LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


OF" 


Class 


J 


V 


AN 


ELEMENTARY    COURSE 


OP 

NATURAL  AND  EXPERIMENTAL 
PHILOSOPHY, 

FOE  THE  USE  OF  HIGH  SCHOOLS  AND  ACADEMIES, 

IN  WHICH 

THE    PKINCIPLES   OF  THE    PHYSICAL    SCIENCES  ARE.   FAMILIARLY 

EXPLAINED    AND    ILLUSTRATED    BY    NUMEROUS 

EXPERIMENTS  AND  DIAGRAMS. 

BYT.  TATE,  F.R.A.S., 

OF    KNELLER    TRAINING    COLLEGE,    ENGLAND. 
AMERICAN    EDITION,    REVISED    AND    IMPROVED, 

BY  C.  S.  CARTEE,  A.M., 

PRINCIPAL  OF  HARVARD  SCHOOL,   CHARLESTOWN. 


BOSTON : 

HICKLING,    SWAN,    AND    BROWN 
1856. 


Entered,  according  to  Act  of  Congress,  in  the  Year  1856,  by 

HICKLING,  SWAN,  AND  BROWN, 
In  the  Clerk's  Office  of  the  District  Court  of  the  District  of  Massachusetts. 


ELECTROTYPED  AT  THE 
BOSTON    STEREOTYPE    FOUNDRY. 


EDITOR'S  PREFACE. 


NOTWITHSTANDING  the  number  and  variety  of  text  books 
on  Natural  Philosophy  now 'before  the  public,  the  peculiar 
excellences  of  Professor  Tate's  treatise,  together  with  the 
expressed  desire  of  some  of  our  most  eminent  teachers  that  it 
might  be  made  available  to  them  in  their  instructions,  seemed 
to  call  for  its  republication  in  this  country.  It  was  originally 
prepared  as  one  of  a  series  of  text  books  for  the  use  of  the 
masters  and  students  of  the  Battersea  College,  near  London ; 
and  how  well  it  has  fulfilled  its  purpose  is  evident  from  the 
high  commendations  bestowed  upon  it  by  Her  Majesty's  In- 
spectors of  Schools,  and  the  Committee  of  Council  on  Edu- 
cation. 

As  a  text  book  for  schools,  it  appears  to  embrace  all  the 
essential  requisites.  The  principles  of  the  science  are  clearly 
and  concisely  stated,  with  their  practical  application  to  the  arts 
of  life  and  the  phenomena  of  nature.  The  experiments  are 
instructive  and  interesting,  requiring  for  the  performance  of 
most  of  them  but  a  trifling  expenditure  for  apparatus.  Ex- 
amples and  problems  to  exercise  the  student,  with  numerous 
diagrams  for  illustration,  pervade  the  work. 

The  article  on  Astronomy,  with  the  use  of  the  globes,  is  a 
small  but  very  useful  introduction  to  one  of  the  most  interesting 
sciences ;  the  illustrations  are  numerous  and  good ;  some  of 

11 '191'  ^ 


4  EDITOR  S    PREFACE. 

them  unique,  and  most  of  them  of  a  superior  order ;  the  model 
lesson,  showing,  by  a  suggestive  course  of  reasoning,  how  the 
student  may  be  led  to  reflect  and  reason  upon  the  knowk-dge 
communicated  to  him,  will  be  read  with  interest  by  those 
engaged  in  teaching. 

In  addition  to  the  subjects  usually  embraced  in  a  course  on 
Natural  Philosophy,  it  contains  an  article  on  Experimental 
Chemistry,  which  is  a  familiar  exposition  of  the  principles  of 
•the  science,  with  their  practical  application  to  agriculture. 
This  must  greatly  enhance  the  value  of  this  manual  in  those 
schools  where  time  and  means  will  not  allow  the  use  of  a  more 
extended  treatise.  £> 

Such  alterations  in  the  ^arrangement  of  the  matter  and  in 
modes  of  expression,  together  with  important  additions  to  the 
text  of  the  original,  have  been  made,  as  were  considered  neces- 
sary to  adapt  it  to  use  in  this  country.  A  series  of  questions 
is  appended  to  the  work  for  the  accommodation  of  those  who 
may  desire  to  use  them. 

c.  s.  c. 


CONTENTS. 


PAGE 

LIST  OF  CHEMICALS  AND  APPARATUS,    .          .          .          .          .10 

INTRODUCTION  :  — 

Four  classes  of  the  laws  of  nature,       .  .  .  .  11 

MECHANICS  :  — 

Laws  of  matter,        .......      11 

Properties  of  matter,      .....  13 

Gravity, .17 

Laws  of  motion,  ...... 

Laboring  forces,        .  .  .  .  .  .  .32 

Mechanical  powers,         ......  35 

Wheelwork, 47 

Exercises,  .......  52 

THE  STEAM  ENGINE:  — 

Different  pieces  of  mechanism  connected  with,      .            .  .52 

The  steam  boiler  and  its  appendages,    ....  58 

Different  forms  of  the  steam  engine,          .            .            .  .61 

High  and  low  pressure  engines,             ....  65 

HYDROSTATICS  :  — 

Properties  of  fluids,              .            .  ...            .            .71 

Levelling.  —  Fluids  transmit  pressure  equally  in  all  directions,          73 

Specific  gravity. — Experiments,  .            .            .            .            80 

Hydrostatic  balance.  —  Hydrometer,  .            .            .            .84 

Floating  bodies,  &c.,      .  .                                   .            87 

1*  (5) 


6  CONTENTS. 

HYDRAULICS  :  — 

Velocity  with  which  water  spouts  from  a  vessel,  .  .-  .90 

Springs  and  artesian  wells,      .....  92 

Hydraulic  machines,  ......        93 

Exercises  on  hydrostatics  and  hydraulics,      ...  95 

PNEUMATICS  :  — 

Properties  of  air, .98 

Pressure  of  the  air,       .  .  100 

Elasticity  of  the  air,          .  .  .  .  .  .105 

Variation  in  the  density,  .....  107 

Air  pump,   .  .°          .  .  .  .  .  .  HO 

Pneumatic  and  hydraulic  machines,    ....  116 

Diffusion  of  gases.  —  Experiments,          ....  121 

Acoustics. —Experiments,      *.....  122 

Transmission  of  sound.  —  Reflection,      ....  124 

Winds,  causes  of,          ......  128 

Trade  winds,  monsoons,  variables,  ....  129 

Balloons. — Exercises  on  pneumatics,  .  .  .  132 

LIGHT  :  — 

Sources  of  light,  &c.,    '     .  .  .  .  .  .135 

Experiments  on  leading  principles  of  optics,  .  .  138 

Reflection  of  light,  .  .  .  .  .  .144 

Refraction  of  light,       .  .  .  .  .  .  148 

Focal  distance  of  lenses,  &c.,        .....      151 

Optical  instruments.  — The  eye,  the  microscope,  the  telescope,  &c.,  154 
Phenomena  of  color,     ......  167 

Unusual  refraction  of  light,  .....      171 

Polarized  light, 174 

HEAT  :  — 

Experiments  elucidating  simple  principles,          .  .  <.  177 

Laws  of  heat,  ......  186 

Propagation  of  heat,          ......  189 

Capacities  of  bodies  for  heat,  ....  195 


CONTENTS.  7 

Liquefaction,  vaporization,  &c.,                ....  197 

Meteorology,      .......  203 

ELECTRICITY  — 

Preliminary  views  and  experiments,         ....  205 

Conductors  and  non-conductors,          ....  212 

Electroscopes,  —  Theories,            .....  214 

Conduction  and  induction,        .....  218 

Electrical  machines,           .           .            .           .           .            .  224 

Attraction  and  repulsion,         .            .           .           .           .  231 

Luminous  effects,    .......  235 

Mechanical  effects,        ......  238 

Peculiar  application  of  the  principle  of  induction,          .           .  240 

Atmospheric  electricity,           .            .            .            .            .  265 

Different  modes  of  generating  electricity,            .           .           .  271 

MAGNETISM  :  — 

Magnetic  power.  —Attraction,            ....  276 
Magnetic  polarity.  —  Theory,        .           .           .           .            .280 

Induction  and  conduction,                                             '.  285 
To  magnetize  steel  bars,  &c.,         .            .           .            .            .290 

Terrestrial  magnetism,             .....  295 

VOLTAIC  ELECTRICITY:  — 

'  Voltaic  pile,  &c.  —  Preliminary  views,      ....  303 

Voltaic  batteries.  —  Voltameters,        ....  309 
Effects  of  voltaic  electricity,          .            .            .           .            .319 

ELECTRO-DYNAMICS  :  — 

Electro-magnetism,      .            .            .           .            .            .  327 

Action  of  electric  and  magnetic  currents,             .            .            .  333 

Motions  produced  by  the  mutual  action  of  magnets  and  currents,  338 

Electro-dynamic  induction,       ....  341 

Thermo-electricity,              ......  348 

Action  of  electro-magnets  upon  different  bodies,        .            .  349 

Electro-magnetic  telegraph,           .....  3oO 

Telegraph  lines  in  the  United  States,              .            .  354 


8  CONTENTS. 

ASTRONOMY  :  — 

Objects  of.  —  General  views,         .           .           .           .           .  355 

Solar  system,      .......  362 

The  earth  and  its  motions,             .....  363 

Lines  upon  the  globe,               .....  368 

Annual  motion  of  the  earth.  —  Seasons,              .           .           .  373 

The  moon.  —  Eclipses  of  moon  and  sun,  375 

The  sun  and  planets,          ......  382 

Comets,              .......  390 

Planets  move  in  ellipses,    ......  391 

Atmospheric  refraction,            .            .                       .  394. 
Twilight.—  Tides.  —Fixed  Stars,           .           .           .            .399 

Division  of  time,            ......  402 

Model  exercises,      .......  465 

THE  USE  OF  THE  GLOBES  :  - 

The  terrestrial  globe.  — Definitions,  &c.,        ...  417 

Problems,    ........  425 

The  celestial  globe. —Definitions,  &c.,     *      .           .           .  443 
Problems,             '    .            .           .            .           .           .            .444 

EXPERIMENTAL  CHEMISTRY  :  — 

Section  I.  —  Nature  of  chemistry.  Simple  and  compound  bodies. 
Different  kinds  of  attraction.  Chemical  affinity.  Nature  of 
acids  and  alkalies.  Solutions,  ....  449 

Section  II.  —  Familiar  experimental  illustratigns  of  the  proper- 
ties and  compounds  of  some  of  the  most  important  simple 
substances,  .....  i  .  455 

Section  III.  —  Metals  and  metallic  oxides,     .  .  .  468 

Section  IV.  —  Doctrine  of  equivalents.  Table  of  equivalents  and 
symbols.  Chemical  nomenclature,  ....  477 

Section  V.  —  Experiments  conducted  "on  a  larger  scale,  or  with 
more  complete  apparatus,  ....  482 

Section  VI.  —  Composition  of  vegetable  substances.  Compound 
organic  substances  in  plants.  Fermentation.  Vegetable 
acids.  Germination.  Structure  and  functions  of  plants. 
Food  of  plants,  .  .  .  .  .  .500 


CONTENTS.  9 

Section  VII.  —  Composition  of  soils.  Their  physical  character. 
Their  origin.  Their  mechanical  properties.  Chemical  prop- 
erties, .......  507 

Section  VIII.  —  Improvement  of  soils.  Mechanical  operations  : 
draining,  ploughing,  &c.  Manuring :  vegetable,  animal,  and 
mineral  manures.  Special  manures.  Rotation  of  crops. 
Fallowing.  Irrigation,  .  .  .  .  .612 

QUESTIONS,       ........  520 


LIST    OF    CHEMICALS    AND    APPARATUS    ADAPTED    TO    THE 
EXPERIMENTS    CONTAINED    IN   THIS    TREATISE. 


List  of  Chemicals. 


Ac 


d,  Arsenious. 


Hydrochloric. 
Nitric. 
Oxalic. 
Sulphuric. 
Tartaric. 
Alum. 

Ammonia,  Liquid,  concentrated. 
"          Carbonate. 
"         Hydrochlorate. 
"          Oxalate. 
Antimony,  Metallic. 

"          Sulphuret. 
Barium,  Chloride. 
Baryta,  Nitrate. 
Camphor. 

Clay,  Pipe,  for  luting. 
Copper  Leaf. 
"       Nitrate. 
"      Sulphate. 
Distilled  Water. 
Gold  Leaf. 
Iodine. 

Iron,  Sulphate. 
"     Sulphuret. 
Lead,  Acetate. 

"      Oxide,  Litharge. 


Lime,  Hydrochlorate. 
Litmus. 

Magnesia,  Sulphate. 
Manganese,  Black  Oxide,  in  pow- 
der. 
Mercury. 

"        Chloride,  (corrosive  sub- 
limate. 
"        Nitrate. 
Phosphorus. 

Platinum,  Wire  and  Sheet. 
Potassa,  fused  in  pipes. 
Carbonate. 
Chlorate. 
Bichromate. 
Nitrate. 
Prussiate. 
Potassium. 

Silver,  Nitrate,  Crystals. 
Soda  Carbonate. 
"     Sulphate. 

Spirit,  Pyroligneous,  for  spirit  lamp. 
Sulphur,  Sublimed. 
Tin  Foil. 
Tincture  Galls. 
"       Litmus. 
"      Red  Cabbage. 


List  of  Apparatus. 


Bladders,  plain  and  mounted. 

Cork  Borers,  set  of  five. 

Crucibles,  fire-clay. 

Evaporating  Basins. 

Filtering  Paper. 

Flasks,  plain,  and  with  tubes  for 

making  gases. 
Furnace  Iron,  with  chimney. 

"  "     with  sand  bath,  &c. 

"        Chauffers. 
Funnels,  Glass. 

Gas  Jars,  plain,  stopped  and  capped. 
"    Holder,  small. 
"         "        with      Oxy-hydrogen 
Blowpipe. 

Ladles,  Iron,  for  supporting  ignited 
Phosphorus,  &c. 


Lamps,  Spirit. 

"      Oil,  with  chimney. 
Mortar  and  Pestle,  Wedgewood. 
Pneumatic  Trough. 
Test  or  Precipitating  Glasses. 
Retort  Stands,  iron,  three  rings. 
Retorts,  glass,  plain  and  tubulated. 
Retort,  iron,  for  making  oxygen. 
Receivers,  glass. 
Scales  and  Weights. 
Stirring  Rods,  glass. 
Stop-cocks,  brass. 

jets. 

"          connectors. 
Test  tubes. 


(10) 


NATURAL  AND  EXPERIMENTAL 
PHILOSOPHY. 


INTRODUCTION. 

1.  THE  general  laws  of  nature  are  divisible  into  the  four 
classes   of,   I.    PHYSICS,  often   called   Natural   Philosophy; 
II.    CHEMISTRY;    III.    LIFE,  commonly  called  Physiology; 
and,  IV.   MIND. 

2.  The  la-ws  of  PHYSICS  govern  every  phenomenon  of  nature  in  which 
there  is  any  sensible  change  of  place. 

3.  The  great  physical  truths  are  reduced  to  four,  and  are  referred  to 
by  the  terms  Atom,  Attraction,  Repulsion,  and  Inertia. 

4.  Solid  bodies  existing  in  conformity  with  these  truths,  exhibit  all 
the  phenomena  of  Mechanics ;   Liquids   exhibit  those  of  Hydrostatics 
and  Hydraulics ;  Airs  those  of  Pneumatics ;  and  Imponderables  those  of 
Heat,  Light,  Electricity,  and  Magnetism. 


MlftHANICS. 

LAWS    OF   MATTER   AND    MOTION. 

5.  MECHANICS,  in  its  most  comprehensive  sense,  treats  of 
the  laws  of  rest  and  motion  of  material  bodies.  Statics  treats 
of  the  equilibrium  of  solid  bodies,  and  Dynamics  treats  of  the 
motion  of  solid  bodies.  Hydrostatics  treats  of  the  equilibrium 
of  fluid  bodies,  and  Hydro-dynamics  (Hydraulics)  treats  of 
the  motion  of  fluid  bodies. 

(11) 


12  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

6.  MATTER  is  known  to  us  by  its  properties,  which  affect 
our  senses.     The  mass  of  a  body  is  the  quantity  of  matter 
which  it  contains.     The  density  of  a  body  is  the  comparative 
quantity  of  matter  contained  in  a  given  size  or  volume. 

7.  MOTION.     A  body  is  in  motion  when  it  is  in  the  act  of 
changing  its  place. 

When  a  body  passes  over  equal  spaces  in  equal  successive  portions 
of  time,  its  motion  is  said  to  be  uniform.  When  the  successive  spaces 
described  in  equal  times  continually  increase,  the  motion  is  said  to  be 
accelerated;  and  when  those  spaces  continually  decrease,  the  motion  is 
said  to  be  retarded.  Motion  is  uniformly  accelerated  or  retarded  when 
the  increase  or  decrease  of  the  spaces  passed  over  in  equal  successive  por- 
tions of  time  is  always  equal. 

8.  VELOCITY.    The  velocity  of  a  body  is  measured  by  the 
space  uniformly  passed  over  in  a  given  time. 

When  the  motion  of  a  body  is  accelerated  or  retarded,  the  velocity  is 
not  measured  by  the  space  actually  passed  over  in  a  given  time,  but  by 
the  space  which  would  have  been  passed  over  in  the  given  time  if  the 
motion  had  continued  uniform  from  that  point. 

9.  MOMENTUM.    The  momentum  of  a  body  is  its  quantity 
of  motion,  and  is  measured  by  the  weight  of  the  body  multiplied 
by  its  velocity. 

The  quantity  of  motion,  or  momentum,  of  a  small  body  may  be  as 
great  as  that  of  a  large  body ;  for  example,  if  the  velocity  of  a  musket 
ball  be  100  times  the  velocity  of  a  heavy  hammer,  which  is  100  times 
the  weight  of  the  ball,  then  their  momenta,  or  quantities  of  motion,  will 
be  the  same.  The  deficiency  of  weight  in  the  ball  is  made  up  by  its 
excess  of  velocity. 

When  a  person  running  strikes  agains^n  obstacle,  he  suffers  a  collision 
corresponding  to  his  weight  and  the  speed  at  which  he  is  moving. 

If  two  bodies  moving  in  the  same  direction  come  into  collision  with 
each  other,  the  force  of  collision  is  measured  by  the  difference  of  their 
momenta ;  but  if  they  are  moving  in  opposite  directions,  the  force  of 
collision  is  much  greater,  for  it  is  equal  to  the  sum  of  their  momenta. 
Hence  it  is  that  the  collision  of  railway  trains,  when  moving  in  opposite 
directions,  is  much  more  terrific  than  when  they  are  moving  in  the  same 
direction. 

10.  FORCE  is  that  which  produces,  or  tends  to  produce, 
motion  in  a  body ;  or  it  is  that  which'  changes  the  uniform 


PROPERTIES    OF    MATTER.  13 

and  rectilinear  motion  of  a  body.     Thus  pressure,  impulse, 
gravity,  &c.,  are  called  forces. 

When  a  force  acts  only  for  an  instant,  it  is  called  impulsive ;  and  when 
it  acts  without  intermission,  it  is  called  a  constant  force.  Constant  forces 
may  be  either  uniform  or  variable.  A  force  is  uniform  when  it  always 
produces  equal  effects  in  equal  successive  portions  of  time ;  and  it  is 
variable  when  the  effects  produced  in  equal  portions  of  time  are  unequal. 

11.  Matter  is  either  ponderable  or  imponderable.     Pon- 
derable  bodies   have   an   appreciable    weight;    imponderable 
bodies  comprise  those  subtile  fluids  which  have  no  appreciable 
weight,  such  as  light,  heat,  magnetism,  and  electricity. 

12.  Forces  are  known  to  us  only  by  the  effects  which  they 
produce. 

In  order  to  estimate  the  magnitude  of  forces,  we  must  compare  the 
effects  which  they  produce  under  the  same  circumstances.  A  force  may 
be  estimated  by  the  pressure  which  it  produces  'upon  some  obstacle  ;  or 
it  may  be  estimated  by  the  motion  which  it  produces  in  a  body  in  a 
given  time.  In  the  former  case  the  measure  of  the  force  is  said  to  be 
statical,  and  in  the  latter  case  dynamical. 


PROPERTIES    OF    MATTER. 

13.  The    properties   of  matter   are   usually  divided   into 
primary,  or  essential,  and  secondary,  or  non-essential. 

The  former  are  those  without  which  we  cannot  conceive  matter  to 
exist ;  the  latter  are  those  which,  depending  upon  the  particular  laws 
impressed  upon  different  substances,  do  not  necessarily  enter  into  our 
abstract  conceptions  of  matter ;  thus,  for  example,  had  it  pleased  the 
Creator,  the  law  of  gravitation  might  have  been  different  from  what  it  is ; 
or,  in  the  place  of  the  law  of  perfect  elasticity,  observed  in  some  bodies, 
all  the  forms  of  matter  might  have  been  practically  incompressible. 
It  is  obvious,  therefore,  that  the  secondary  properties  of  matter  could  not 
have  become  known  to  us  anterior  to  observation  and  experiment.  The 
relative  adaptation  of  these  secondary  properties  of  matter  to  the  condi- 
tions and  constitution  of  the  universe,  affords  the  most  striking  evidence 
of  the  existence  and  attributes  of  a  great  and  intelligent  cause. 

14.  The  primary  properties  of  matter  are  Extension  and 
Impenetrability.     The  most'  important?  secondary  properties, 

2 


14  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

considered  in  relation  to  mechanical  science,  are  Compressi- 
bility, Expansibility,  Divisibility,  Cohesion,  Elasticity,  Mobility, 
Inertia,  and  Gravity. 

15.  EXTENSION   is   that   property   whereby   every   body 
must  occupy  a  certain  limited  space.    We  necessarily  conceive 
every  body  to  have  length,  breadth,  and  thickness. 

16.  IMPENETRABILITY  is  that  property  whereby  no  two 
substances  can  occupy  the  same  space  at  the  same  instant  of 
time. 

17.  COMPRESSIBILITY  and  EXPANSIBILITY  are  those  prop- 
erties by  virtue  of  which  bodies  may  be  made  to  occupy  a 
smaller  or  larger  space. 

The  susceptibility  to  compression  shows  that  all  bodies  must  contain 
pores,  or  spaces  between  the  ultimate  particles  or  atoms  of  which  they 
are  composed,  and  that  there  is  no  substance  in  nature  which  is  absolutely 
solid. 

In  consequence  of  these  properties,  bodies  differ  very  much  in  density. 
When  bodies  have  the  same  size,  or  volume,  their  densities  are  measured 
by  their  weights.  Thus  a  cubic  foot  of  copper  weighs  nine  times  as 
much  as  a  cubic  foot  of  water ;  hence  copper  possesses  nine  times  the 
density,  or  specific  gravity,  of  water. 

18.  DIVISIBILITY.     There  is  no  limit  to  the  mathematical 
conception  of  the  divisibility  of  space  :  but  the  doctrine  of 
the  atomic  theory  seems  to  indicate  that  there  is  a  practical 
limit  to  the  divisibility  of  matter. 

In  going  on  with  our  division,  we  must  finally  arrive  at  a  certain 
ultimate  particle,  or  atom  of  matter,  which,  from  its  constitution,  no 
longer  admits  of  separation  into  parts.  Nature  presents  us  with  various 
marvellously  minute  divisions  of  the  particles  of  matter. 

19.  COHESION,  or  the  attraction  of  cohesion,  is  that  prop- 
erty of  bodies  whereby  the  atoms  composing  them  are  united 
in  a  mass. 

This  force  of  attraction  between  the  particles  of  matter  only  takes 
place  at  immeasurably  minute  distances.  Bodies  are  solid,  liquid,  or 
aeriform,  according  as  the  cohesion  of  their  particles  is  modified  by  heat. 
The  particles  of  gases  and  vapors  repel  one  another,  in  consequence  of 
the  repulsive  force  of  heat*being  greater  than  the  force  of  cohesion ;  in. 


PROPERTIES    OP   MATTER.  15 

solids,  the  force  of  cohesion  preponderates  over  that  of  repulsion ;  and 
in  liquids  the  forces  of  cohesion  and  repulsion  are  presumed  to  be  equal. 

20.  ELASTICITY  is  that  property  of  bodies  by  which,  when 
their  form  is  altered  by  the  action  of  an  external  force,  they 
regain  their  original  form  as  soon  as  the  external  force  is 
withdrawn. 

All  bodies  possess  this  property  in  a  greater  or  less  degree. 

Most  substances  have  a  limit  to  their  elasticity :  thus,  if  a  straight 
elastic  bar  is  bent  by  a  pressure  applied  to  it,  and  if  this  pressure  does 
not  exceed  a  certain  quantity,  the  bar  will  resume  its  original  form  when 
this  pressure  is  removed  ;  but,  on  the  contrary,  if  the  pressure  exceeds  a 
certain  quantity,  called  the  limit  of  the  body's  elasticity,  the  cohesion 
of  the  material  is  injured  or  destroyed ;  and  then,  in  this  case,  the  bar 
will  not  return  to  its  original  form  upon  the  cessation  of  the  pressure. 
Bodies  which  have  no  elastic  limit  may  be  called  perfectly  elastic,  such 
as  gases  and  vapors. 

Liquids  scarcely  admit  of  compression ;  and  hence  they  are  called 
non-elastic  fluids,  whereas  gases  and  vapors  are  called  elastic  fluids. 
Some  aeriform  bodies,  such  as  carbonic  acid  gas,  have  been  brought  to 
the  liquid  state  by  being  subjected  to  a  high  pressure  and  cold ;  these  are 
called  condensable  gases ;  whereas  some  gaseous  bodies,  such  as  oxygen 
and  nitrogen,  composing  the  atmosphere,  resist  condensation,  whatever 
may  be  the  pressure  and  cold  to  which  they  are  subjected.  These  gases 
are  called  permanently  elastic.  Beams  employed  in  construction  are 
sometimes  considered  perfectly  elastic,  when  their  resistance  to  compression, 
within  then-  limits  of  elasticity,  is  equal  to  their  resistance  to  extension.  - 

21.  MOBILITY,  or  susceptibility  to  motion,  is  that  property 
whereby  a  body  admits  of  change  of  place. 

Motion  may  be  absolute  or  relative  :  thus  a  man  in  a  railway  carriage 
may  be  in  motion  relatively  to  the  other  objects  in  the  carriage,  while  at 
the  same  time  he  partakes  of  the  absolute  motion  of  the  train.  In  esti- 
mating motion,  there  are  three  things  to  be  considered ;  viz.,  the  velocity 
or  quickness  of  the  motion,  the  space  passed  over,  and  the  time  in  which 
that  space  is  passed  over.  The  motion  of  a  body  is  uniform  when  it 
passes  over  equal  space  in  equal  successive  intervals  of  time ;  in  this  case, 
the  velocity  of  the  motion  is  the  distance  in  feet  passed  over  in  one 
second  of  time  ;  the  space  is  the  whole  distance  in  feet  moved  over  ;  and 
the  time  the  number  of  seconds  in  which  the  space  is  described.  In 
uniform  motion,  therefore,  we  have 

The  space  =  the  velocity  X  the  time. 


16  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Here  there  are  three  general  quantities,  any  two  of  which  being  given, 
the  remaining  one  may  be  found. 

EXAMPLES 

Ex.  1.    If  a  railway  train  moves  over  44  feet  in  a  second,  what  space 
will  it  move  over  in  an  hour  ? 

Space  moved  over  in  1  sec.  =  44  ft. 

"      3600  sec.  or  1  h.  =  3600  X  44  ft.  =  158,400  ft. 
=  30  miles.     Ans. 

Ex.  2.    If  a  railway  train  moves  over  20  miles  in  an  hour,  what  will 
be  its  velocity  per  second  ? 

Space  moved  over  in  3600  sec.  or  1  h.  =  20  X  5280  ft. 


Or,  in  1  sec.  =  =  29  J  ft.     Ans. 


Ex.  3.  If  the  velocity  of  a  body  be  20  feet  per  second,  in  what  time 
will  it  move  over  a  mile  ? 

5280  ft.  .  o 

2Q   ft  =264  sec.  =  4|  mm.    Ans. 

22.  INERTIA.  By  this  property  is  meant  that  matter  has 
no  power  in  itself  to  change  its  present  state,  and  that  any 
alteration  in  its  state,  whether  of  rest  or  motion,  must  be  pro- 
duced by  the  action  of  some  external  force. 

If  a  body  is  broken,  some  force  must  have  produced  the  rupture.  If 
a  body  is  melted,  heat  must  have  produced  the  change.  If  a  body 
changes  its  state  from  rest  to  motion,  some  force  must  have  communi- 
cated the  motion.  If  it  passes  from  a  state  of  motion  to  that  of  rest, 
some  force  must  have  been  exerted  to  destroy  the  motion.  The  laws  of 
motion  will  be  hereafter  more  fully  considered. 

Experiment.  Place  a  penny  on  a  piece  of  card  paper,  and  balance  it 
upon  the  tip  of  one  of  the  fingers  of  the  left  hand,  as  shown  in  Fig.  1  ; 


Fig.  \. 
give  the  card  a  smart  blow  with  one  of  the  fingers  of  the  right  hand ; 


PROPERTIES    OF   MATTER.  17 

the  card  will  be  projected  forward,  but  the  penny,  from  its  inertia,  will 
remain  on  the  finger. 

When  a  carriage  suddenly  stops,  the  person  in  it  is  liable  to  be  thrown 
forward,  from  the  inertia  of  his  body ;  that  is,  in  this  case,  from  the 
tendency  which  his  body  has  to  continue  in  motion. 

A  body  in  motion  also  tends  to  move  forward  in  a  straight  line; 
hence  the  effort  we  have  to  make  when  we  run  round  a  corner.  When 
a  stone  is  whirled  round  in  a  sling,  it  flies  off  in  a  direct  course  the 
moment  it  is  allowed  to  escape.  It  is  well  known  that  a  hare,  acting  by 
instinct  on  this  law  of  inertia,  sometimes  makes  its  escape  from  the 
greyhound  by  taking  a  great  many  sudden 
turns,  which  the  dog,  from  its  greater  bulk 
and  inertia,  does  not  so  readily  take.  Thus, 
in  running  to  the  cover  C,  (see  Fig.  2,)  the 
hare  takes  the  course  A  B  D  E  C,  while  the 
dog  is  compelled  to  take  the  longer  course, 
A  b  d  e  C. 

When  the  equestrian,  standing  on  the  sad- 
dle, leaps  over  a  cord  extended  over  the 
horse  at  right  angles  to  his  motion,  the  horse 
passes  under  the  cord  while  the  rider  leaps 
over  it,  and  lights  on  the  saddle  at  the  oppo- 
site side.  Here  the  equestrian  has  merely  to 
leap  upwards,  not  forwards,  as  he  would  have  to  do  if  he  were  not  in 
motion ;  for  while  in  the  act  of  leaping  he  retains  the  motion  which  he 
had  before  he  made  the  leap,  so  that,  when  he  arrives  at  the  opposite  side 
of  the  cord,  his  progressive  motion  being  the  same  as  that  of  the  -horse, 
he  lights  exactly  on  the  saddle. 

23.  GRAVITY  is  that  property  by  which  all  terrestrial 
bodies  tend  towards  the  centre  of  the  earth.  When  a  body 
is  supported,  this  tendency  produces  pressure  and  weight. 

The  pressure  produced  by  gravity  is  always  exerted  in  a  direction  per- 
pendicular to  the  horizon,  and  is  measured  by  the  weight  of  the  body. 
The  unit  of  weight  in  mechanical  calculations  is  a  pound ;  and  hence 
the  forces  of  pressure  are  usually  expressed  in  units  of  pounds. 

It  has  been  found  by  experiment  (allowance  being  made  for  the  resist- 
ance of  the  air)  that  bodies  of  every  size,  shape,  and  weight  fall  to  the 
earth  exactly  in  the  same  manner.  Thus,  were  it  not  for  the  resistance 
of  the  air,  a  feather  and  a  guinea  would  fall  from  the  top  of  a  tower  in 
the  same  time,  and  they  would  strike  the  ground  with  the  same 
velocity. 

2* 


18  NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

Experiment.  Take  a  small  piece  of  thin  paper  and  a  coin,  and  let 
them  fall  the  same  instant  from  equal  heights  above  the  ground  ;  then 
the  coin  will  arrive  at  the  ground  much  sooner  than  the  paper.  Here 
the  air  presents  a  greater  proportional  resistance  to  the  motion  of  the 
light  body  than  it  does  to  the  heavy  one  ;  but,  in  order  that  the  resist- 
ance of  the  air  may  be  the  same  in  both  bodies,  place  the  paper  on  the 
coin,  and  then  let  them  fall  together ;  they  both -arrive  at  the  ground  at 
the  same  instant. 

Thus  it  appears  that  the  pressure  produced  by  the  earth's  attraction 
upon  bodies  is  a  very  different  thing  from  the  motion  which  it  generates. 
In  the  former  case,  the  pressure  produced  is  proportional  to  the  quantity 
of  matter ;  whereas,  in  the  latter  case,  the  motion  generated  in  a  given 
time  is  the  same  for  all  bodies,  whatever  may  be  their  size,  weight,  or 
density.  This  admits  of  a  satisfactory  explanation :  the  earth  attracts 
every  particle  of  which  a  body  is  composed,  and  hence  the  weight  of  a 
body  depends  upon  the  matter  which  it  contains.  On  the  other  hand, 
all  the  particles  of  a  body  separated  from  one  another  would  evidently 
fall  through  the  same  spaces  in  the  same  time ;  but  it  appears  from  ex- 
periment, that  when  the  particles  are  collected  in  one  mass,  they  fall 
exactly  in  the  same  manner  as  they  would  if  they  were  separated  from 
one  another. 

24.  Gravity  is  said  to  take  place  in  consequence  of  the 
attraction  exerted  by  the  earth  upon  the  body.  The  gen- 
eral name  given  to  this  force  is  that  of  attraction  of  grav- 
itation. 

This  force  is  not  confined  to  bodies  upon  the  earth's  surface ;  the 
moon  is  maintained  in  her  orbit  by  the  attraction  of  the  earth,  and  all 
the  planetary  bodies  in  the  solar  system  are  subject  to  the  attraction  of 
the  sun. 

The  attractive  force  exerted  by  bodies  on  each  other  is 
reciprocal,  and  in  proportion  to  their  masses. 

Thus,  if  the  body  A  attracts  the  body  B,  then  B  will  attract  A,  and 
the  forces  which  they  exert  on  each  other  will  be  proportional  to  their 
respective  masses. 

Again,  the  force  of  attraction  varies  inversely  as  the  square 
of  the  distance. 

Thus  at  double  the  distance  the  force  will  be  one  fourth,  at  treble 
one  ninth,  and  so  on. 

These  two  laws  are  expressed  by  saying  that  the  force  of 


PROPERTIES    OF   MATTER.  19 

gravitation  varies  directly  as  the  mass,  and  inversely  as  the 
square  of  the  distance. 

Bodies  are  attracted  by  the  earth  as  if  the  whole  of  its  mass  were  col- 
lected in  its  centre  ;  hence  the  force  of  gravity  at  any  place  depends  upon 
the  distance  of  the  place  from  the  centre  of  the  earth.  Now,  since  the 
equatorial  diameter  of  the  earth  is  greater  than  the  polar  diameter,  it 
follows  that  the  force  of  gravity  at  places  near  the  equator  is  not  so 
great  as  it  is  at  places  near  the  poles ;  thus  it  is  found  that  a  body  which 
produces  by  its  gravity  a  certain  pressure  at  Boston,  would  not  produce 
this  amount  of  pressure  if  taken  to  the  equator  ;  and  in  like  manner  a 
pendulum  which  beats  seconds  at  Boston  would  take  a  longer  time  to 
complete  a  vibration  at  the  equator. 

In  consequence  of  the  constant  action  of  the  force  of  gravity,  the 
motion  of  a  falling  body  becomes  quicker  and  quicker  as  it  descends. 
The  velocity  acquired  by  a  falling  body  in  one  second  is  32^  feet,  in  two 
seconds  it  is  twice  32  £,  in  three  seconds,  it  is  three  times  32^,  and  so  on. 
That  is,  the  velocity  acquired  by  a  falling  body  increases  ivith  the  time  ; 
or,  in  other  words,  the  velocity  acquired  by  a  falling  body  in  feet  is  equal 
to  the  product  of  32^  feet  by  the  number  of  seconds  of  its  fall. 

Experiment.     If  a  body  takes  three  seconds  in  falling  from  the  top  of 
a  tower,  with  what  velocity  will  the  body  strike  the  ground  ? 
Velocity  =  3  X  32£  ft.  =  96^  ft. 

This  law  of  acquired  velocity  arises  from  the  fact  that  gravity  is  a 
uniformly  accelerating  force,  communicating  equal  increments  of  velocity 
in  equal  times,  and  that  each  successive  increment  of  velocity  is  unaf- 
fected by  the  motion  previously  acquired.  At  places  towards  the  equa- 
tor the  accelerating  force  of  gravity  is  less  than  it  is  in  our  latitude,  and 
at  places  near  the  poles  it  is  greater.  The  laws  of  descending  bodies 
will  hereafter  be  more  fully  considered. 

25.  CENTRE  OP  GRAVITY.  The  centre  of  gravity  of  a 
body  is  that  point  in  it  where  all  the  matter  composing  it  may 
be  supposed  to  be  collected.  The  centre  of  gravity  of  any 
regular  body  lies  in  its  centre  of  magnitude. 

Balance  a  rod  or  a  stick,  or  any  other  body,  upon  the  finger ;  that 
point  upon  which  the  body  is  balanced  is  the  centre  of  gravity.  If  the 
centre  of  gravity  of  a  body  be  supported,  the  body  will  remain  at  rest ; 
and  in  all  other  positions  the  centre  of  gravity  descends  to  the  lowest 
place  to  which  it  can  get. 

A  vertical  line  drawn  through  the  centre  of  gravity  of  a  body  is  called 


20 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


the  line  of  direction.     If  the  line  of  direction  fall  within  the  base,  the 
body  will  stand;  if  not,  it  will  fall.     Thus, 


P 
Fig.  5.  Fig.  6. 

let  G  G,  &c.,  be  the  centres  of  gravity  of  four  bodies  standing  on  their 
horizontal  bases,  and  G  P,  G  P,  &c.,  the  lines  of  direction;  then  the 
bodies  represented  in  Figs.  3  and  4  will  stand,  because  the  lines  of  direc- 
tion fall  within  their  bases.  The  body  represented  in  Fig.  5  will  be  upon 
the  point  of  falling,  because  the  line  of  direc- 
tion just  falls  at  the  edge  of  its  base ;  and  the 
body  represented  in  Fig.  6  will  fall,  because 
the  line  of  direction  falls  without  its  base. 

When  a  man  carries  a  load  upon  his  back, 
he  leans  forward,  to  bring  the  centre  of  gravi- 
ty of  his  body  and  the  load  which  he  carries 
within   the  base  formed  by  his  feet.     If  he 
were  not  to  do  so,  the  load  would  be  liable 
to  draw  him  over  backward.      For  the 
same  reason,  when  a  man  walks  up  a  hill 
he  leans  forward,  and  when  he  descends 
he  leans  backward. 

A  cylinder  may  be  made  to  roll  up  an 
inclined  plane.  Fix  a  piece  of  lead,  s,  in 
one  side  of  the  cylinder  z  ;  then  it  will 
roll  up  the  inclined  plane  to  the  position 


Fig.  8. 


Fig.  9. 

z' s',  because  the  centre  of  gravity  of  the  mass  will  endeavor  to  descend  to 
its  lowest  point. 


PROPERTIES    OF   MATTER. 


21 


Fig.  10. 


If  a  body  be  supported  by  a  point  lying  above 
its  centre  of  gravity,  the  body  is  said  to  be  sus- 
pended, and  if  it  be  free  to  move,  it  will  not  rest 
until  its  centre  of  gravity  has  attained  the  low- 
est possible  position.  Thus,  for  example,  if  the 
ball  K  be  suspended  by  the  thread  s  a,  it  will  not 
rest  until  its  centre  of  gravity,  s,  attains  the  lowest 
possible  position,  that  is,  in  this  case,  when  4he 
thread  hangs  vertically. 

Fig.  1 1  shows  how  two  forks  may  be  suspended 
on  the  point  of  a  needle.  Stick  two  forks,  A  and 
B,  into  a  cork,  C;  then  stick  a  sewing  needle,  -with  its  point  outwards, 
into  the  cork,  and  poise  the  whole  on  the  top  of  a  wine  glass,  or  on  the 
head  of  a  pin  stuck  into  another  cork.  Here  the 
stability  of  the  system  depends  upon  the  fact,  that 
the  centre  of  gravity  is  below  the  point  of  support. 

In  like  manner  a  fork  may  be  suspended  over  the 
edge  of  the  table  on  the  point  of  a  needle,  as  shown 
in  Fig.  12.  Here  the  point  of  suspension,  P,  lies  in 
the  vertical  Hue,  P  C,  passing  through  the  centre  of 
gravity,  C,  of  the  fork. 

To  find  the  centre  of  gravity  of  any  plane  sur- 
face, suspend  it  freely  by  any  point,  and  draw  the 
line  of  direction  through  that  point  of  suspension  ; 
suspend  the  surface  by  another  point,  and  in  like  manner  draw  the  line 
of  direction  through  it ;  then  the  intersection  of  these  two  lines  will 
give  the  centre  of  gravity  of  the  surface. 

Of  all  forms  of  structure,  having  the  same  height  and 
base,  the  pyramidal  form  is  the  strongest.  The  pyra- 
mid represented  in  Fig.  13,  which  stands  on  a  broad 
base,  is  more  stable  than  that  represented  in  Fig.  14, 
which  has  a  narrow  base ;  because  the  centre  of  gravity, 
G,  must  be  raised  through  a  greater  space  in  the  former 
case  than  in  the  latter  case,  before  they  can  be  over- 
turned. 

The  body,  ABC,  (Fig.  15,)  has  a  position  of  stable 
equilibrium,  because  its  centre  of  gravity,  C,  has  attained 
its  lowest  possible  position  ;  whereas  the  body  represented  in  Fig.  16  has 
a  position  of  unstable  equilibrium,  because  its  centre  of  gravity,  C,  has 
not  attained  its  lowest  possible  position  ;  the  slightest  force  will  cause  its 
centre  of  gravity  to  descend,  and  to  occupy  the  position  represented  in 
Fig.  15. 

A  cart  loaded  with  stone  may  pass  safely  along  a  road  of  which  one 


Fig.  12. 


22 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


side  is  higher  than  the  other  ;  but  if  the  same  cart  were  loaded  with  hay, 
it  would  be  overturned ;  for,  though  the  sustaining  base  be  the  same  in 


B  0 

Fig.  13. 

both  cases,  the  fine  of  direction  falls  much  within  it  from  the  low  centre 
of  gravity  of  the  stone,  but  very  near  the  wheel,  or  altogether  on  the 
outside,  from  the  high  centre  of  the  hay. 

The  feet  of  our  common  chairs,  and  of  tripods,  are  generally  expanded 
below  to  give  a  broad  base.  The  high  chair,  to  accommodate  the  little 
child  at  the  dining  table,  is  very  dangerous  if  the  feet  do  not  spread 
much. 

The  famous  leaning  tower  of  Pisa  is  believed  to  have  been  purposely 
so  built.  Its  height  is  130  feet ;  and  though  its  top  overhangs  the  base 
16  feet,  its  centre  of  gravity  is  within  the  base. 

The  upright  form  of  man  stands  firmly  on  a  very  narrow  base,  which 
is  the  space  occupied  by  his  feet.  The  advantage  of  turning  out  the  toes 
is,  that  without  taking  much  from  the  length  of  the  base,  it  adds  to  its 
breadth. 

A  person  on  rising  from  a  chair  first  bends  the  body  forward  so  as  to 
bring  the  feet  under  the  centre  of  gravity,  and  then  lifts  the  body. 

When  a  man  walks  at  a  moderate  rate,  his  centre  of  gravity  comes 
alternately  over  the  right  and  over  the  left  foot,  causing  the  body  to 
advance  in  a  waving  line.  Persons  walking  arm  in  arm  jostle  each 
other,  unless  the  movement  of  their  feet  correspond  as  do  those  of  sol- 
diers in  inarching. 


LAWS   OF   MOTION. 

26.  FIRST  LAW  OF  MOTION.  A  body  in  motion  will 
move  continually  in  a  straight  line,  and  with  a  uniform  veloci- 
ty, if  it  is  not  acted  on  by  any  external  force. 

Many  persons  are  apt  to  think  that  a  body  in  motion  would  stop  of 
itself;  but  this  is  not  correct,  for  it  is  only  the  obstacles  which  a  body  in 
motion  meets  with  that  causes  it  to  stop.  Thus,  when  a  body  is  rolled 
along  a  floor,  the  f notion  of  the  floor  causes  the  body  to  come  to  a  state 


LAWS    OF    MOTION.  23 

of  rest ;  but  we  know  that  the  smoother  the  floor  the  farther  will  the 
body  roll.  The  resistance  of  the  air  also  tends  to  stop  bodies  in  motion. 
Hence  it  is  that  a  wheel  with  vanes  will  revolve  much  longer  in  the  ex- 
hausted receiver  of  an  air  pump  than  it  will  do  in  the  open  atmosphere. 
Gravity  also  tends  to  destroy  motion :  a  body  thrown  upward  soon  loses 
its  motion,  and  returns  to  the  earth's  surface. 

"Whenever,  therefore,  a  body  in  motion  comes  to  a  state  of  rest,  we 
may  safely  infer  that  some  external  force  or  resistance  has  checked  the 
motion ;  and  that  a  body  in  motion  would  never  stop,  that  is  to  say,  it 
would  move  on  and  on  in  a  straight  line  for  ever,  if  it  did  not  meet  with 
any  external  force  or  resistance  to  stop  it. 

SECOND  LAW  OP  MOTION.  If  any  number  of  forces 
act  at  the  same  instant  upon  a  body  in  motion,  each  force  pro- 
duces its  full  effect  in  the  direction  of  its  action,  just  as  if  it 
had  acted  alone  upon  the  body  at  rest. 

Thus,  if  a  ball  be  dropped  from  the  top  of  the  mast  of  a  ship  moving 
uniformly,  the  ball  strikes  the  deck  at  the  bottom  of  the  mast,  and  falls 
precisely  in  the  same  time  as  if  the  ship  were  at  rest. 

Although  the  earth,  by  its  diurnal  motion,  carries  all  bodies  on  its 
surface  uniformly  from  west  to  east,  yet  all  motions  take  place  on  the 
earth's  surface  just  as  if  it  were  at  rest. 

If  a  ball  be  thrown  along  the  deck  of  a  vessel  moving  uniformly,  it 


5 

Fig.  17. 

will  move  on  the  deck  in  precisely  the  same  manner  as  if  the  vessel  were 
at  rest.  Let  S  represent  the  deck  of  the  vessel  moving  uniformly  in  the 
water.  Suppose  the  vessel  to  move  from  S  to  s,  or  that  the  point  A  moves 
from  A  to  C  in  the  same  time  that  the  ball  moves  from  A  to  B.  Now, 
whilst  the  ball  is  moving  on  the  line  A  B,  across  the  deck,  it  is  at  the 
same  time  carried  with  the  vessel  from  A  to  C,  and  at  the  end  of  the 
time  the  ball  is  found  at  D  ;  so  that  it  preserves  its  two  motions ;  that  is 
to  say,  it  moves  in  the  direction  A  B  as  it'  it  had  no  other  motion,  and  in 
the  direction  A  C  with  the  vessel,  as  if  it  had  no  other  motion.  The 
actual  path  pursued  by  the  ball  is  evidently  in  the  diagonal,  A  D,  of  the 
parallelogram,  A  B  D  C. 

This  establishes  what  is  called  the  parallelogram  of  motion,  which 
may  be  enunciated  as  follows  :  — 


24 


NATURAL    AND    EXPERIMENTAL     PHILOSOPHY. 


PARALLELOGRAM  OF  MOTION.  If  two  velocities  be  given 
to  a  body  at  the  same  instant,  the  actual  velocity  will  be  rep- 
resented by  the  diagonal  of  the  parallelogram  formed  upon 
the  two  lines  representing  the  velocities  impressed  upon  the 
body. 

Let  a  body  at  A  (Fig.  17)  have  a  Telocity  given  to  it  which  would 
cause  it  to  move  uniformly  from  A  to  C  in  a  given  time,  and  another  ve- 
locity at  the  same  instant,  which  would  cause  it  to  move  uniformly  from 
A  to  B  in  the  same  time.  Now,  if  the  parallelogram,  A  B  C  D,  be  com- 
pleted, the  actual  path  of  the  body  will  be  the  diagonal,  A  D,  described  in 
the  same  time. 

When  a  boatman  is  rowing  his  boat  n c 

(Fig.  18)  across  a  strong  stream,  the  boat 
has  two  distinct  impulses  given  to  it ;  the 
impulse  given  by  the  man,  which  tends  to 
carry  the  boat  directly  across  the  stream, 
from  A  to  B,  and  that  of  the  stream  itself, 
which  tends  to  carry  the  boat  along  with 
it  from  A  to  D.  Under  the  action  of  these 

two  simultaneous  impulses  the  boat  moves  in  the  direction  of  the  diago- 
nal, A  C. 

Very  nearly  allied  to  the  parallelogram  of  motion  is  the  parallelogram 
offerees. 

The  parallelogram  of  forces  is  this :  if  the 
sides  A  D  and  A  B  (see  Fig.  19)  of  the  par- 
allelogram, A  B  C  D,  represent  the  magni- 
tude and  direction  of  two  forces  acting  at  the 
same  moment  on  the  body,  A,  then  the  diago- 
]pal,  A  C,  will  represent  the  magnitude  and 
direction  of  the  resultant  force,  or  the  single 
force  which  the  two  forces  acting  together 
produce. 

Thus,  if  the  body,  A,  be  pressed  in  the  direction  A  B  with  a  force  of  3 
pounds,  and  at  the  same  time  in  the  direction  A  D  with  a  force  of  4 
pounds,  then  these  two  forces  acting  together  will  produce  a  single  force 
Avhose  direction  and  magnitude  may  be  readily  found  by  constructing 
the  parallelogram  of  forces.  From  any  scale  of  equal  parts  take  A  B, 
equal  to  3  units,  representing  the  force  in  the  direction  A  B  ;  from  the 
same  scale  take  A  D,  equal  to  4  units,  representing  the  force  in  the  direc- 
tion A  D  ;  construct  the  parallelogram,  A  B  C  D  ;  then  the  diagonal,  A  C, 
will  be  the  direction  of  the  single  resulting  force,  and  the  units  in  A  C 
will  be  the  magnitude  of  this  force,  viz.,  5  pounds. 


LAWS    OF   MOTION.  25 

THIRD  LAW  OF  MOTION.  Action  and  reaction  are  al- 
ways equal  and  contrary. 

If  a  person  presses  the  table  with  his  finger,  he  feels  a  resistance  arising 
from  the  reaction  of  the  table ;  and  this  counter-pressure  is  equal  and 
contrary  to  the  downward  pressure.  "When  a  horse  draws  a  load  for- 
ward, he  is  pulled  backward  by  the  load.  "When  a  gun  is  fired,  the 
explosion  of  the  powder,  which  gives  the  forward  motion  to  the  ball,  at 
the  same  time  gives  the  recoil  to  the  gun.  "When  a  bird  flies,  it  strikes 
the  air  downward  with  its  wings,  and  thereby  produces  a  reaction  suffi- 
cient to  support  it  in  the  atmosphere.  If  a  man  in  a  boat  pull  another 
boat  towards  him,  by  means  of  a  rope,  then,  from  the  law  of  action  and 
reaction,  both  boats  will  move  towards  each  other  in  such  manner  that 
their  momenta  shall  be  equal. 

If  an  elastic  ball  be  projected  in  a  direction  perpendicular  to  the  sur- 
face of  a  hard  pavement,  the  reaction  will  cause  the  ball  to  rebound  in 


Fig.  20. 

the  direction  in  which  it  was  projected.  Now,  if  the  ball  be  projected 
obliquely,  it  will  rebound  obliquely,  making  the  angle  of  reflection  equal 
to  the  angle  of  incidence. 

The  intensity  of  the  action  of  any  force  is  estimated  by  the  mass  and 
velocity  of  the  body  which  it  sets  in  motion ;  that  is  to  say,  by  the  mo- 
mentum of  the  body  which  it  sets  in  motion.  Thus,  if  a  cannon  ball 
be  fifty  times  the  wreight  of  a  musket  ball,  but  the  musket  ball  be  moved 
with  fifty  times  the  velocity  of  the  cannon  ball,  then  both  balls  will  have 
the  same  momentum,  and  will  strike  any  obstacle  with  the  same  force. 
Again,  let  A  and  B  be  two  bodies  in  motion ;  A  weighs  8  Ibs.,  and  moves 
with  the  velocity  of  3  feet  per  second ;  B  weighs  4  Ibs.,  and  move  with 
the  velocity  of  6  feet  per  second ;  then 
wt.  X  velo. 

8    X    3  =  24,  momentum  A. 

4    X    6  =  24,  momentum  B. 

that  is  to  say,  the  momenta,  in  this  case,  are  equal,  and  the  quantities 
of  motion  in  them  are  equal,  and  the  intensity  of  the  forces  producing 
these  motions  are  equal. 
3 


26  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

If  a  body  in  motion  impinges  upon  another  body,  the  quantity  of  mo- 
tion, or  momentum,  of  the  two  bodies  after  impact  will  be  the  same  as  it 
was  before  impact :  the  momentum  lost  by  the  one  body  is  exactly  the 
same  as  that  which  is  gained  by  the  other  body ;  and  this  is  true  whether 
the  bodies  be  elastic  or  non- elastic. 

Ex.  1.  Let  A  and  B  be  two  non-elastic  bodies  moving  in  the  same  di- 
rection, and  that  A  impinges  upon  B  ;  let  the  weight  of  A  be  6  Ibs.,  and 
its  velocity  8  feet  per  second,  and  the  weight  of  B  2  Ibs.,  and  its  velocity 
4  feet  per  second ;  required  the  velocity  with  which  the  two  bodies  will 
move  on  together  after  impact. 

Here  the  momentum  of  A  before  impact  =  6  X  8  =  48  ; 

Momentum  of  B  before  impact  =  2X4  =  8; 

Momentum  of  mass  after  impact  =  48  -f-  8  =  56. 

Now,  as  the  bodies  are  non-elastic,  they  will  move  on  together,  after 
impact,  with  the  same  velocity.  But  the  common  velocity  of  the  two 
bodies  will  be  found  by  dividing  their  momentum  by  the  sum  of  their 
weights,  which,  in  this  case,  is  6  Ibs.  -f-  2  Ibs.  =  8  Ibs. 

Velocity  of  the  bodies  after  impact  =  —  =  7  ft.  per  sec.    Any. 

Ex.  2.  Required  the  same  as  in  the  last  example  when  the  bodies 
move  in  opposite  directions. 

In  this  case  the  momentum  of  B  must  be  subtracted  from  the  momen- 
tum of  A ;  thus, 

Momentum  after  impact  =  48  — .8  =  40; 

Telocity  of  the  bodies  after  impact  =  -g-  =  5  ft.  per  sec.    Ans. 

When  the  bodies  are  elastic,  the  case  is  somewhat  different ;  for  they 
do  not  move  on  together  after  impact  with  a  common  velocity,  owing  to 
the  reaction  of  the  elastic  material  of  which  the  bodies  are  composed. 

The  equality  of  action  and  reaction  in  the  collision  of  bodies  may  be 
illustrated  by  the  following  simple  experimental  apparatus  :  A  and  B  are 
two  balls  suspended  by  equal  strings,  A  C  and  B  C,  c 

so  that  the  balls  may  be  in  contact  with  each  other  ; 
E  F  is  a  graduated  arc,  of  which  C  is  the  centre, 
over  which  the  balls  may  oscillate.  One  of  the 
balls,  A,  is  drawn  aside  along  a  certain  number  of 
the  arc,  and  then  allowed  to  fall  and  strike  the  other 
ball,  B,  which  will,  in  consequence  of  the  collision, 
move  up  the  other  portion  of  the  arc.  The  velocity  A  B 

with  which  A  impinges  upon  B  is  measured  by  the  j.-     ^i 

number  of  degrees  of  the  arc  through  which  it  falls, 
and  the  velocity  of  the  bodies  after  impact  is  measured  by  the  number 
of  degrees  of  the  arc  through  which  they  ascend. 


EFFECTS    OF    GRAVITY. 


27 


Exp.  3.  Let  the  two  balls  be  composed  of  soft  clay,  or  any  other  non- 
elastic  substance ;  then  after  impact  they  will  move  on  together  with  a 
common  velocity,  which  may  be  calculated,  as  in  Ex.  2. 

Suppose  the  balls  to  be  equal  in  weight,  and  that  A  impinges  upon  B 
at  rest ;  then  the  two  balls  will  move  together  with  a  velocity  due  to  that 
which  A  had  at  the  moment  of  impact.  And  so  on  to  other  cases,  which 
may  be  readily  verified  by  experiment. 

Let  the  two  balls  be  composed  of  ivory,  or  any  other  substance  which 
is  nearly  perfectly  elastic,  and  let  them  be  of  the  same  size.  Suppose 
the  ball  A  to  impinge  upon  the  ball  B  at  rest ;  then  after  impact  A  will 
remain  at  rest,  and  B  will  move  on  with  the  same  velocity  as  A  had  at 
the  moment  of  impact.  In  this  case  the  reaction  of  elasticity  causes  the 
ball  A  to  stop,  and  the  ball  B  to  move  forward  with  the  motion  which  A 
had  at  the  instant  of  impact.  And  so  on  to  other  cases,  which  may  ba 
readily  verified  by  experiment. 


EFFECTS    OF   GRAVITY. 


Falling  Bodies. 

27.  It  has  already  been  explained  that,  since  gravity  is  a  constantly 
acting  force,  it  causes  bodies  to  fall  quicker  and  quicker  in  the  course  of 


Space  in  1  sec. 


Space  in  2  sec. 


Space  in  3  sec. 
32X16TJft  -1- 


Velocity  acquired  in 
leec. 


Velocity  acquired  in 
2  sec. 


Fig.  22. 


28  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

their  descent,  and  that  the  velocity  acquired  at  any  instant  is  propor- 
tional to  the  time  of  descent.  Now,  in  1  second  a  body  falls  through  16  ^ 
feet;  in  2  seconds  it  falls  through  4  times  IGj-1^  feet,  or  64^-  feet;  in  3 
seconds  it  falls  through  9  times  16 ^  feet,  or  144f  feet;  and  so  on,  the 
law  of  descent  being  as  follows :  the  space  passed  over  by  a  fatting  body 
is  equal  to  IGrj  feet  multiplied  by  the  square  of  the  numb&r  of  seconds 
during  which  the  body  has  been  fatting.  Thus  the  space  moved  over  in 
3  seconds  is  equal  to  32  X  16-^-  feet  =  144f  feet,  and  the  space  moved 
over  in  4  seconds  is  equal  to  42  X  16rV  feet  ==  257^-  feet ;  and  so  on. 

Fig.  22  shows  the  relation  between  the  tune,  space,  and  velocity  ac- 
quired by  a  falling  body. 

A  glance  at  Fig.  22  will  show  that  the  spaces  fallen  through  in  each 
successive  second  are  as  the  numbers  1,  3,  5,  7,  &c.;  that  is  to  say,  for 
example,  the  space  fallen  through  during  the  3d  second  will  be  equal  to 
5  times  16T^  ft.,  or  80TV  ft. 

Ex.  1.  Through  what  space  will  a  body  fall  in  5  sec.  ? 

Ans.  402T12-  ft. 

Ex.  2.  Through  what  space  will  a  body  fall  in  2J-  sec.  ? 

Ans.  lOOf  f  ft. 

Ex.  3.  What  space  will  a  body  descend  during  the  4th  second  of  its 
fall?  Ans.  112r73-ft. 

Ex.  4.  In  what  time  would  a  body  acquire  a  velocity  of  160|  ft.  ? 

Ans.  5  sec. 

When  a  body  is  projected  vertically  upward,  its  motion  is 
uniformly  retarded,  and  it  will  rise  to  the  same  height  as  that 
from  which  it  would  have  to  fall  in  order  to  acquire  the  ve- 
locity of  projection. 

Thus,  for  example,  if  the  body  be  projected  vertically  upward  with  a 
velocity  of  3  times  32<|,  the  force  of  gravity  will  destroy  all  its  motion 
in  3  seconds,  so  that  the  height  to  which  it  will  rise  will  be  equal  to 
32  X  16TV  ft.  =  144f  ft. 

Ex.  1.  If  a  body  be  projected  vertically  upward  with  a  velocity  of 
193  ft.  per  second,  to  what  height  will  it  ascend?  Ans.  579  ft. 

Ex.  2.  If  a  body  be  projected  vertically  upward  with  a  velocity  of 
64^-  ft.,  in  what  time  will  it  return  to  the  ground?  Ans.  4  sec. 

Projectiles. 

28.  When  a  body  is  projected  obliquely  in  the  air,  it  de- 
scribes a  curved  line,  which  is  called  a  parabola. 


EFFECTS    OF    GRAVITY.  29 

Were  it  not  for  the  force  of  gravity,  the  body,  according  to  the  first 
law  of  motion,  would  move  uniformly  on  in  the  direction  of  the  straight 
line  in  which  it  is  projected  ;  but  the  force  of  gravity  causes  it  to  be  de- 
flected from  this  straight  line ;  so  that,  under  the  combined  action  of  the 
force  of  projection  and  that  of  gravity,  the  body  moves  in  a  curved 
line.  When  the  body  reaches  the  highest  point,  it  descends  in  a  curve 
which  is  exactly  the  same  as  the  curve  which  it  pursued  in  its  ascent. 


iv  -^ 


Fig.  23. 


Let  a  body  be  projected  in  the  line  a  b  (see  Fig.  23,)  with  a  velocity 
which  would  carry  it  (if  gravity  were  not  acting)  from  a  to  I  in  1  sec- 
ond, from  a  to  II  in  2  seconds,  and  so  on ;  then  the  path  of  the  body  will 
be  in  the  parabola  a  d  efg,  where  c  is  the  highest  point  of  ascent,  and 
the  curve  efgoi  descent  has  the  same  form  as  the  curve  a  d  e  of  ascent. 
The  path  of  the  projectile  may  be  found  in  the  following  manner  :  — 
Draw  the  vertical  a  c;  take  a  h  =  16 73,  the  space  through  which  a 
3* 


30  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

body  will  fall  in  1  second ;  a  I  —  4  X  16iV>  the  sPace  through  which  a 
body  will  fall  in  2  seconds;  a  k  =  9  X  16iV»  the  space  through  which 
a  body  will  fall  in  3  seconds ;  and  so  on :  draw  h  d,  I  e,  kf,  &c.,  parallel 
to  a  b,  and,  intersecting  the  verticals  drawn  through  the  points  I,  II,  III, 
&c.,  in  the  points  dy  e,f,  £c.,  then  the  path  of  the  projectile  will  be  in 
the  curve  a  d  efg. 

The  Pendulum. 

29.  The  times  of  the  vibrations  of  the  pendulum  are  very 
nearly  equal,  whether  it  be  moving  much  or  little  ;  that  is  to 
say,  whether  the  arc  described  by  it  be  large  or  small. 

Hence  it  is  employed  to  regulate  the  machinery  of  our  clocks.  The 
time  which  a  pendulum  takes  to  make  a  vibration  depends  upon  its 
length ;  it  is  well  known  that  the  longer  the  pendulum  the  greater  is  the 
time  which  it  takes  to  perform  a  vibration.  It  has  been  ascertained  that 
the  lengths  of  different  pendulums  vary  as  the  squares  of  then*  respective 
times  of  vibrations  :  thus,  a  pendulum  which  vibrates  in  3  seconds  must 
be  nine  times  the  length  of  a  pendulum  which  vibrates  in  1  second ;  a 
pendulum  which  vibrates  in  half  a  second  must  be  a  quarter  the  length 
of  a  pendulum  which  vibrates  in  1  second ;  and  so  on.  The  length  of 
a  pendulum  vibrating  seconds  at  London  is  about  39^  inches,  and  there- 
fore the  length  of  a  pendulum  to  vibrate  half-seconds  must  be  the  quar- 
ter of  39^  inches,  or  about  9f  inches. 

Motion  round  a  Centre. 

30.  When  a  body  moves  round  a  centre,  it  is  acted  upon 
by  two  forces,  viz.,  the  force  of  projection,  which  gives  the 
body  motion,  and  the  centripetal  force,  or  centre-seeking  force, 
which  retains  it  in  its  circular  path,  thereby  preventing  it  from 
flying  off  in  a  straight  line,  or  in  a  tangent  line  to  the  curve. 
This  tendency  to  fly  off  in  a  tangent  is  called  the  centrifugal 
force,  or  centre-flying  force.    This  force  is  counteracted  by  the 
centripetal  force. 

Such  is  the  motion  of  the  planets  round  the  sun,  and  the  satellites 
round  then-  respective  primaries.  The  gravitation  of  the  planets  towards 
the  sun  is  the  centripetal  force,  and  the  force  of  projection  we  assume  to 
have  been  at  first  given  to  the  various  planets  by  the  hand  of  the 
Creator. 

One  of  the  most  familiar  instances  of  motion  round  a  centre  is  the 


EFFECTS    OF    GRAVITY. 


31 


whirling  motion  given  to  a  stone  in  a  sling.  Here  the  propelling  force, 
or  the  force  of  projection,  is  given  by  the  hand,  and  the  centripetal  force 
is  exhibited  in  the  tension  of  the  string ;  when  we  quit  the  string,  the 
centripetal  force  no  longer  acts,  and  the  stone,  by  the  action  of  the  cen- 
trifugal force  generated  by  the  whirling  motion,  flies  off  at  a  tangent. 

"When  we  whirl  a  mop,  the  water  flies  from  it  by  the  action  of  the 
centrifugal  force,  and  the  threads  of  the  mop  assume  the  form  of  a  sphe- 
roid, or  of  a  sphere  flattened  at  the  poles  of  revolution.  In  like  manner 
the  earth  is  a  great  globe  flattened  at  the  poles.  The  rotation  of  the 
earth  upon  its  axis  has  caused  the  equatorial  parts  to  bulge  out. 

When  a  carriage  is  moved  rapidly  round  a  corner,  it  is  very  liable  to 
be  overturned  by  the  centrifugal  force  thus  brought  into  action. 

"When  an  animal  moves  round  in  a  circle,  he  leans  towards  the  centre, 
in  order  to  counteract  the  centrifugal  force.* 

When  railways  form  a  rapid  curve,  the  outer  rail,  D,  (Fig.  24,)  is  laid 
higher  than  the  inner  rail,  E,  in  order  to  counteract  the  effect  of  the  cen- 


Fig.  24. 

trifugal  force,  which,  acting  through  the  centre  of  gravity,  G,  of  the  car- 
riage, lias  a  tendency  to  throw  it  off  the  line.    The  rise,  K  D,  of  the  outer 
rail  will  of  course  depend  upon  the  quickness  of  the 
curve  and  the  breadth  of  the  rail. 

The  following  instructive  experiment  is  sometimes 
performed  by  conjurers  :  A  B  is  a  hoop  which  re- 
volves upon  an  axis,  O  ;  W  is  a  wine  glass  of  water 
placed  within  the  hoop.  Now,  when  a  rapid  motion 
of  rotation  is  given  to  the  hoop,  the  wine  glass  of 
water  is  sustained  in  its  place  by  the  centrifugal  force 
that  is  thus  generated ;  and  if  the  experiment  be  care- 
fully made,  not  a  single  drop  of  water  will  be  thrown 
from  the  glass. 


Fig.  25. 


32  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


LABORING   FORCES. 

31.  When  work  is  performed  by  any  agent,  there  is  always  a  certain 
weight  or  resistance  moved  over  a  certain  space.  The  amount  of  work 
done  will  obviously  depend  upon  the  weight  or  resistance  that  is  moved, 
and  the  space  over  which  it  is  moved.  In  order  to  estimate  the  amount 
of  work  done  by  any  laboring  force,  it  is  requisite  that  we  should  fix 
upon  some  unit  of  icork.  Now,  the  unit  of  work  adopted  in  this  coun- 
try is  the  labor  expended  in  raising  a  pound  weight  one  foot  high  in  op- 
position to  gravity  ;  or,  what  amounts  to  the  same  thing,  it  is  the  labor 
expended  in  moving  a  resistance  of  one  pound  through  the  space  of 
one  foot  in  opposition  to  the  direction  in  which  the  resistance  acts.  From 

this  definition  of  a  unit  of  work  it  follows,  that 

* 

The  work  expended  in  raising  any  body  in  opposition  to 
gravity  is  equal  to  the  product  of  its  weight  in  pounds  by  the 
vertical  space  in  feet  through  which  it  is  raised. 

For  example,  the  work  expended  in  raising  50  Ibs.  to  the  height  of  20 
feet  will  be  equal  to  50  X  20  =  1000. 

In  calculating  the  work  requisite  to  pump  water  from  a  mine,  it  is 
only  necessary  that  we  should  find  the  weight  of  the  water  in  pounds, 
and  then  multiply  this  result  by  the  depth  of  the  mine  in  feet. 

When  a  horse  draws  a  carriage  along  a  road,  the  work  which  he  per- 
forms is  expended  in  overcoming  the  resistance  of  the  friction  of  the  road 
to  the  motion  of  the  carriage.  Now,  on  any  given  road,  this  resistance 
of  friction  is  simply  proportional  to  the  weight  of  the  load ;  so  that,  in 
calculating  the  work,  we  allow  so  many  pounds'  resistance  for  every  ton 
weight  in  the  load.  The  work  in  this  case  will  be  found  by  multiplying 
the  total  resistance  of  friction  in  pounds  by  the  space  in  feet  over  which 
the  carriage  is  moved. 

It  is  also  customary  to  express  work  in  units  of  a  horse  power.  Watt 
estimated  that  a  horse  could  perform  33,000  units  of  work  per  minute  ; 
this  work,  therefore,  is  called  a  horse  power.  In  order,  therefore,  to 
determine  the  number  of  horse  powers  of  an  engine  requisite  for  per- 
forming a  certain  amount  of  work,  we  must  first  find  the  number  of 
units  of  work  which  must  be  done  per  minute,  and  then  divide  this 
result  by  33,000  to  find  the  number  of  horse  powers. 

EXAMPLES. 

Ex.  1.  How  many  horse  powers  would  it  take  to  raise  2  cwt.  of 
coals  per  minute  from  a  pit  whose  depth  is  100  fathoms  ? 

Weight  of  the  coals  in  Ibs.  =  2  X  112  =  224  ; 


LABORING   FORCES.  33 

Depth  of  the  pit  in  feet,  =      6  X  100  =         600 ; 
Work  to  be  done  per  min.,  =  224  X  600  =  134.400  ; 

134,400 

No.  of  horse  powers,  =  -^-r^  =  4.07,  Ans. 
GO, 000- 

Ex.  2.  Required  the  same  as  in  the  last  example,  when  the  weight 
of  the  coals  is  1  cwt.,  and  the  depth  of  the  pit  is  400  fathoms. 

Ans.  8.14. 

Ex.  3.  How  many  horse  powers  would  be  required  to  raise  1000 
cubic  feet  of  water  per  hour  from  a  mine  whose  depth  is  ninety  fathoms  ? 

Weight  of  water  in  Ibs.  =  62.5  X  1000  =  62,500  Ibs. ; 

Depth  of  the  mine  in  feet  =6X90  =  540  feet ; 

Work  to  be  done  per  hour  =  62,500  X  540  ; 

62,500  X  540 
Work  to  be  done  per  min.  = — =  62,500  X  9  ; 

62,500  X  9 
No.  of  horse  powers  =  — 33Q  QQ  -  =  17,  Ans. 

Ex.  4.  Required  the  same  as  in  the  last  example,  when  the  number 
of  cubic  feet  of  water  =  1250,  and  the  depth  of  the  mine  =  43  fath- 
oms. Ans.  10.1. 

Ex.  5.  If  a  man  can  perform  2500  units  of  work  per  minute,  in 
what  time  will  he  pump  100  cubic  feet  of  water  from  a  well  whose  depth 
is  500  feet  ? 

Work  to  be  done  =  100  X  62.5  X  500  ; 

100  X  62.5  X  500  1250 

Tune  to  do  the  work  = 25QQ ==  125°  minutes,  =  -^~ 

=  20.83  hours,  Ans. 

Ex.  6.  Required  the  same  as  in  the  last  example,  when  th$  number 
of  cubic  feet  of  water  =  50,  and  the  depth  of  the  well  =  250  feet. 

Ans.  5.2  hours. 

Ex.  7.  What  must  be  the  horse  powers  of  a  locomotive  engine  which 
moves  at  the  steady  speed  of  30  miles  per  hour,  on  a  level  rail,  the  weight 
of  the  train  being  25  tons,  and  the  resistance  of  friction  at  the  rate  of 
8  Ibs.  for  every  ton  ? 

Total  resistance  of  friction  =  8  X  25  =  200  Ibs ; 

30  X  5280 

Distance  this  resistance  is  moved  over  in  ft.  per  mm.  = — 

=  2640  feet ; 

Work  to  be  done  every  minute  =  200  X  2640  ; 

200  X  2640 
Horse  powers  of  the  engine  to  do  this  work  =  — Og  QQQ — ~  =16  horse 

powers,  Ans. 

Ex.  8.  Required  the  same  as  in  the  last  example,  when  the  speed  = 
25  miles,  and  the  weight  of  the  engine  =  60  tons. 

Ans.  32  horse  powers. 


34  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

General   Views  relative  to  Machines. 

32.  The  object  of  machinery,  properly  so  called,  is  to  regulate  the  dis- 
tribution, or  change  the  direction  of  work,  not  to  increase  it.  If  there 
were  no  friction  or  any  other  resistances  to  the  motion  of  the  pieces  com- 
posing a  machine,  the  work  that  would  be  given  out  would  be  exactly 
equal  to  the  work  applied.  Dead  matter,  by  its  gravity,  produces  pres- 
"sure,  and  by  the  intervention  of  mechanism,  that  pressure  may  be  in- 
creased or  decreased ;  but  work  is  peculiarly  the  production  of  active  or 
living  agents.  To  suppose  that  machines  are  capable  of  augmenting 
work  would  be  endowing  inert  matter  with  a  creative  power — the 
power  of  creating  work. 

In  all  instances  of  labor  performed  by  inanimate  matter,  there  is 
some  active  agent  of  nature,  such  as  heat,  electricity,  or  gravitation, 
which  gives  rise  to  the  work ;  but,  in  the  case  of  merely  mechanical 
arrangements,  'the  inert  matter  is  the  passive  recipient  of  work,  or  the 
channel  through  which  it  flows.  Hence  we  may  lay  it  down  as  a  fun- 
damental axiom  in  mechanics,  that  (abstracted  from  friction  and  the 
resistance  of  the  air)  the  work  done  by  any  machine  is  the  same  as  the 
work  applied.  Now,  as  the  work  is  the  product  of  pressure  and  motion, 
it  follows  that,  if  the  working  point  of  a  machine  moves  more  slowly 
than  the  driving  point,  then  the  pressure  at  the  former  will  be  greater 
than  it  is  at  the  latter.  Thus,  for  example,  if  the  power  applied  to  the 
extremity  of  a  lever  moves  twice  as  fast  as  the  weight  or  resistance  at 
the  other  extremity,  then  the  pressure  of  the  power,  in  order  to  raise  the 
weight,  must  be  only  one  half  of  the  pressure  of  the  weight  or  resist- 
ance, for  then  the  work  applied  by  the  power  would  be  exactly  equal  to 
the  work  done  in  raising  the  weight  or  resistance.  So,  in  like  manner, 
in  any  arrangement  of  wheels  or  pulleys,  if  the  power  applied  moves 
say  nine  times  as  fast  as  the  resistance  or  weight  to  be  raised,  then  the 
pressure  of  the  power,  in  order  to  raise  the  weight,  must  be  only  one 
ninth  of  the  pressure  of  the  weight  or  resistance. 

Thus  it  appears,  from  the  principle  of  the  equality  of  work,  that  where 
the  power  applied  to  a  machine  is  just  able  to  raise  the  weight  or  resist- 
ance, the  power  and  the  weight  will  be  to  each  other  inversely  as  their 
velocities;  or,  in  other  words,  the  weight  moved  will  be  as  many  times 
greater  than  the  power  applied  to  move  it,  as  the  velocity  of  the  power  is 
greater  than  that  of  the  weight.  Now,  the  number  of  times  that  the 
weight  is  greater  than  the  power  is  called  the  advantage  gained  by  the 
machine.  Hence  the  advantage  gained  is  equal  to  the  number  of  times 
that  the  velocity  of  the  power  is  greater  than  that  of  the  weight ;  or,  in 
more  precise  language, 

The    advantage    gained  by  a  machine   is   equal   to  the 


MECHANICAL    POWERS.  35 

velocity   of   the   power  divided  by  the  velocity  of  the  re- 
sistance. 

This  is  sometimes  called  the  principle  of  virtual  velocities.  Practical 
men  express  this  law  by  saying,  "  What  you  gain  in  power  you  lose  in 
speed." 

MECHANICAL    POWERS. 

The  simple  machines,  or  mechanical  potocrs,  as  they  have  been  called, 
—  the  lever,  the  wheel  and  axle,  the  pulley,  the  inclined  plane,  the 
wedge,  and  the  screw,  —  enable  man  to  adopt  any  species  and  speed  of 
power  which  he  can  command,  to  almost  any  work  which  he  has  to 
accomplish.  But,  as  we  have  already  explained,  the  advantage  gained  is 
simply  an  advantage  of  pressure,  not  of  work;  for  what' is  gained  in 
pressure  is  lost  in  speed,  and  therefore  the  actual  amount  of  work  done 
by  means  of  the  mechanical  power  is  neither  increased  nor  decreased  ;  in- 
deed, if  the  friction  of  the  parts  of  the  machine  is  taken  into  account, 
the  work  done  by  it  is  really  less  than  that  which  would  be  done  by  the 
man  laboring  without  the  intervention  of  such  machinery. 

The  Lever. 

33.  The  lever  is  an  inflexible  bar  or  rod,  turning  on  a 
pivot,  which  is  called  the  fulcrum.  It  is  used  for  raising 
heavy  weights  over  a  short  distance. 


No.  1.  No.  2. 

Fig.  26. 

Thus,  P  W,  (Fig.  26)  represents  a  crowbar  or  lever,  "W  theresistance, 
C  the  fulcrum,  and  P  the  point  at  which  the  power  is  applied. 

Fig.  27  represents  a  lever  ;  C,  the  fulcrum  or  centre  of  motion;  P  C, 
the  arm  to  which  the  pressure  of  the  power,  P,  is  applied ;  and  C  W, 
the  arm  to  which  the  pressure  of  the  weight,  or  resistance,  ~W,  is 
applied. 

Now,  when  the  lever  comes  to  the  position  p  wt  the  power.  P,  has 


36 


NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 


Fig.  27. 


moved  over  the  arc  P  p,  while  the 
weight  W  has  moved  over  the  arc 
W  w;  these  arcs,  therefore,  respec- 
tively represent  the  velocities  of 
P  and  W. 

Here,  if  the  arm  C  P  were 
double  the  arm  C  W,  the  velocity 
of  P  would  be  double  that  of  W, 

for  a  double  radius  would  sweep  over  a  double  arc  ;  and  if  the  arm  C  P 
were  three  times  the  length  of  the  arm  C  W,  the  velocity  of  P  would 
be  three  times  that  of  W  ;  and  so  on  :  so  that  the  velocity  of  the  power 
is  as  many  times  the  velocity  of  the  weight  as  the  arm  by  which  the 
power  acts  is  longer  than  the  arm  by  which  the  weight  acts  ;  and  there- 
fore, from  what  has  been  explained,  the  advantage  gained  will  be  found 
by  finding  the  number  of  times  that  the  arm  C  P  is  greater  than  the  arm 
C  W  ;  thus,  if  C  P  be  3  times  the  length  of  C  W,  the  advantage  gained 
will  be  3,  and  a  pressure  of  1  cwt.  at  P  will  raise  a  resistance  or  weight 
of  3  cwt.  at  W.  Again,  if  C  P  =  5  feet,  and  C  W=  £  foot,  then  the 
advantage  gained  will  be  10,  because  5  feet  are  equal  to  10  times  £  foot  ; 
and  so  on  to  other  cases. 

34.  Levers  are  divided  into  three  kinds,  according  to  the 
relative  positions  of  the  power  and  weight  with  respect  to  the 
fulcrum. 

Pig.  28  represents  a  lever  of  the  first    A  c  F        u 

7tind,  where  the  power  P  and  weight  W 
act  on  opposite  sides  of  the  fulcrum  F. 
Fig.  26,  No.  1,  also  represents  a  lever  of 
the  first  Idnd. 

Fig.  29  represents  a  lever  of  the  second 
kind,  where  the  power  P  and  weight  W 


7.28. 


, 

act  on  the  same  side  of  the  fulcrum  F  ;  but  W  is  nearer  to  the  fulcrum 
than  P.    Fig.  26,  No.  2,  also  represents  a  lever  of  the  second  kind. 


Fig.  29. 


Fig.  30. 


MECHANICAL    POWERS. 


37 


Fig.  31. 


Fig.  30  represents  a  lever  of  the  third  kind,  where  the  power,  P,  and 
weight,  "VV,  act  on  the  same  side  of  the  fulcrum,  F  ;  but  P,  in  this  case, 
is  nearer  to  the  fulcrum  than  "W. 

When  a  man  raises  a  ladder  against  a  wall,  (see  Fig.  31,)  he  employs 
a  lever  of  the  third  kind.  In  this  case,  the  fulcrum  is  at  the  foot  of  the 
ladder,  the  power  is  applied  by  the  hand  of  the 
man,  and  the  resistance  is  the  weight  of  the 
ladder  itself,  which  acts  through  its  centre  of 
gravity,  G. 

In  the  lever  of  the  second  kind,  (see  Fig.  29,) 
if  the  arm  A  F,  by  which  the  power  acts,  is  5 
feet,  and  the  arm  B  F,  by  which  the  weight 
acts,  is  2  feet,  then  the  advantage  gained  will 
be  5  -7-  2  =  2£  ;  that  is  to  say,  a  power  of  1 
cwt.  applied  at  A  will  just  balance  a  weight 
of  2£  cwt.  applied  at  B,  and  a  power  of  60  Ibs. 
applied  at  A  will  balance  a  weight  of  2£  times 
60  Ibs.,  or  150  Ibs.,  applied  at  B  ;  and  so  on  to 
other  cases. 

In  the  lever  of  the  third  kind,  (see  Fig.  30,) 
there  is  power  lost ;  for  example,  if  B  F  be 

twice  A  F,  then  a  weight  of  1  cwt.  suspended  at  B  will  require  a 
power,  P,  of  2  cwt.  applied  at  A  to  sustain  it. 

A  poker,  as  it  is  usually  employed  in  stirring  the  fire,  is  an  instance 
of  a  lever  of  the  first  kind ;  where  the  bar  of  the  grate  is  the  fulcrum, 
and  the  resistance  moved  is  the  coal  of  the  fire.  The  clawed  hammer, 
as  it  is  used  in  drawing  out  a  nail,  is  also  a  lever  of  the  first  kind.  The 
nut-cracker,  the  oar,  &c.,  are  levers  of  the  second  kind.  The  fire  tongs, 
the  sugar  tongs,  &c.,  belong  to  levers  of  the  third  kind. 

Wheel  and  Axle. 

35.  This  mechanical  power  is  only  another  form  of  the  lever,  where 
the  power  is  made  to  act  without  intermission.  In  its  most  simple  form, 
it  consists  of  a  horizontal  axle,  A,  (Fig.  32,)  and  large  wheel,  R,  which 
turn  upon  two  pivots  supported  in  gudgeons.  A  cord  wrapping  round 
the  axle,  A,  sustains  the  weight,  W,  and  another  cord  wrapping  round 
the  wheel,  R,  in  a  contrary  direction,  sustains  the  power,  P.  These 
forces  always  act  in  the  direction  of  a  tangent  to  the  circle.  Here  the 
leverage  of  the  power  is  the  radius  of  the  wheel,  and  the  leverage  of  the 
weight  is  the  radius  of  the  axle ;  hence  the  advantage  gained  is  equal 
to  the  number  of  times  that  the  radius  of  the  axle  is  contained  in  the 
radius  of  the  wheel :  thus,  if  the  radius  of  the  wheel  is  24  inches,  and 
4 


38 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


that  of  the  axle  3  inches,  then  the  advantage  gained  would  be  8,  and 
power  of  1  ewt.  applied  to  the  wheel 
would  balance  a  weight  of  8   cwt. 
suspended  from  the  axle. 

36.  The  windlass  is  only  another 
form  of  the  wheel  and  axle,  where  the 
handle  C  B  is  substituted  in  the 
place  of  the  wheel.  In  this  case,  the 
advantage  gained  is  equal  to  the  num- 
ber of  times  that  the  length  of  handle 
is  greater  than  the  radius  of  the  axle  : 

thus,  for  example,  if  the  length  of  the  p.     nn 

handle  is  18  inches,  and  the  radius  of 
the  axle  is  2  inches,  then  the  advantage  would  be  9,  and  a  pressure  of 


Fig.  33. 

60  Ibs.  applied  at  the  handle  would  just  raise  a  weight  W  of  9  times 
60  Ibs.,  or  540  Ibs. 

Combination  of  Wheels  and  Axies. 

37.    In  Fig.  34,  F  is  a  wheel,  to  which  the  power  P  is  applied,  and 
B  C  its  axle,  turning  upon  a  common  axis ;  A  D  another  wheel,  with 
its  axle  E  sustaining  the  weight, 
"W.      The   motion  of   the  axle 
B  C  is  transmitted  to  the  wheel 
A  by  means  of  a  cord. 

To  calculate  the  advantage 
gained,  let  the  radius  of  the 
wheel  F  be  18  inches,  that  of 
its  axle  B  C  2  inches,  the  radius 
of  the  wheel  A  D*  36  inches, 
and  that  of  its  axle  E  3  inches  ; 
then  the  advantage  gained  by 
the  first  wheel  and  axle  will  be  Fig.  34. 


MECHANICAL    POWERS. 


39 


Fig.  35. 


equal  to  18  -f-  2  =  9,  so  that  if  P  be  1  lb.,  A  will  produce  a  force  of  9 
Ibs.  on  the  cord  C  A.  The  advantage  gained  by  the  second  wheel  and 
axle  will  be  36  -r  3  =  12,  so  that  a  force  of  1  lb.  applied  to  the  cord, 
C  A,  will  sustain  a  weight  W  of  12  Ibs. ;  and  therefore  a  weight  of  9 
Ibs.  applied  to  the  cord  C  A  will  sustain  a  weight  W  of  9  times  12 
Ibs.,  or  108  Ibs. ;  so  that  the  total  advantage  gained  will  be  108. 

Cogged   Wheels. 

38.  Let  D  (Fig.  35)  be  a  cogged  or  toothed  wheel,  turning  upon  the 
same  axis  as  the  wheel  C  ;  Q  another  cogged  wheel,  acted  upon  by  the 
former,  and  turning  upon  the  same 

axis  as  the  axle  I.  From  the  wheel 
C,  is  suspended  the  weight  P,  and 
from  the  axle  I,  the  weight  W; 
then  while  P  descends,  the  wheel  C 
and  the  cog  D  will  be  turned  round, 
and  a  corresponding  number  of  teeth 
in  the  cog  Q  will  be  turned  in  a  con- 
trary direction ;  and  thus  the  cord 
I  W  will  be  coiled  up  upon  the  axle 
I,  and  the  weight  "VV  will  be  raised. 

"When  the  radii  of  the  wheels  and  axles  are  given,  the  advantage 
gained  by  this    machine  will  be 
found  in  the  same  manner  as  in  the 
foregoing  combination. 

39.  When  the  .axle  is  placed  in 
a  vertical  position,  and  the  power 
is  applied  by  means  of  bars  or  levers 
inserted  into  the  holes  at  H,  as 
shown  in  Fig.  36,  the  machine  is 
called  a  capstan.     In  this  case^the 
cable  coils  round  the  axle  in  the 
form  of  an  endless    rope,   which 
winds  round  the  lower  part  of  the 
axle,   and    at  the  same  time  un- 
winds from  the  upper  part.    The  axle  is  made  conical,  to  enable  the 
workman  to  shift  the  cable  upwards,  as  it  becomes  necessary. 

40.  The  gib  crane,  represented  in  Fig.  36  a,  is  a  laseful  application  of  the 
wheel  and  axle ;  D  O  is  a  vertical  beam,  resting  as  well  as  turning  upon 
a  pivot  at  its  under  end,  and  supported  in  its  upright  position  by  stays 
in  the  floor,  with  rollers  attached  to  them ;  K  B  is  an.  arm  projecting 
from  the  beam  D  O,  having  a  pulley  B  at  its  extremity ;  the  axes  of 
the  wheel  work  are  supported  by  two  cast  iron  crosses,  bolted  on  each 


Fig.  36. 


40 


NATURAL   AND   EXPERIMENTAL   PHILOSOPHY. 


Fig.  36  a. 

side  of  the  vertical  beam ;  H,  the  winch  or  handle,  turns  a  pinion 
fixed  on  its  axis ;  this  pinion  turns  the  spur  wheel  a,  which  carries 
a  pinion  on  its  axis  ;  then  this  latter  pinion  turns  the  large  wheel 
C,  with  its  barrel  or  axle  A,  round  which  the  chain  is  coiled  ;  this  chain 
passes  over  the  pulley  B,  and  has  a  hook  at  its  extremity  for  laying 
hold  of  the  weight  to  be  raised ;  the  barrel  A  has  a  ratchet  wheel  and 
detent  to  prevent  any  recoil.  As  the  gib  admits  of  being  turned  round 
in  any  direction,  a  weight  raised  from  one  side  of  it  may  be  turned 
round  and  let  down  at  the  opposite  side,  or  at  any  part  within  the  sweep 
of  the  gib.  To  understand  the  construction  of  a  crane,  you  should  go  and 
see  one  at  work. 

The  Pulley. 

41.  When  a  rope  P  A  B  W  passes  over  a  fixed  wheel  C  turning  on 
an  axis,  the  mechanism  is  called  a  pulley.  A  force  pull- 
ing at  the  cord  P  A  causes  the  wheel  C  to  turn  upon  its 
axis  from  the  friction  of  the  cord  on  its  edge ;  and  as  the 
wheel  turns  it  gives  off  cord  equal  in  length  to  the  space 
described  by  its  circumference. 

Here  the  motion  of  P  and  W  must  be  equal;  for, 
whatever  space  P  may  descend,  W  will  ascend  through 
the  same  space.  Moreover,  when  equilibrium  takes  place, 
the  tension  or  stretch  of  the  single  cord  P  A  B  W  must 
be  the  same  in  every  part,  and  the  tension  of  the  portion 
A  P  will  be  the  same  as  the  tension  of  the  portion  B  W ; 
therefore  the  weight  P  must  be  equal  to  the  weight  "W, 
in  order  to  produce  these  equal  tensions. 


Fig.  37. 


THE   PULLET. 


41 


Fig.  38  shows  the  manner  in  which  a 
pulley  is  constructed. 

The  pulley  is  said  to  be  fixed  or  mo- 
vable, according  as  its  block  is  fixed  or 
movable.  There  are  various  combina- 
tions of  pulleys ;  in  all  of  them  a  force 
called  the  power  (P)  is  applied  to  the 
first  string,  and  this  sustains  another 
force,  called  the  weight,  (W,)  applied 
to  the  last  string. 

In  Fig.  39  a  continuous  cord  P  AB  D 
passes  over  a  movable  pulley  C,  and  is 
fixed  to  a  hook  at  D.  The  power  is  applied  at  P  ;  and  the  weight  W 
to  be  raised  is  suspended  from  the  block  of  the  pulley.  Here,  as  W  is 
suspended  by  two  cords  A  P  and  B  D,  each  cord  must  sustain  one 
half  the  weight  —  that  is  to  say,  the  power  will  be  one  half  the  weight, 
or  a  power  of  1  Ib.  will  support  a  weight  W  of  2  Ibs. 


Fig.  38. 


Fig.  39. 


Fig.  40. 


In  Fig.  40  F  is  a  fixed  pulley,  and  C  a  movable  one ;  the  single  or  con- 
tinuous cord  P  K  Q  A  B  D  passes  over  the  wheels  F  and  C,  and  is  fixed 
to  a  hook  D.  If  W,  with  its  pulley  C,  ascend  1  foot,  the  cords  B  D 
and  A  Q  will  each  be  shortened  1  foot,  and  therefore  the  cord  K  P  will 
be  lengthened  2  feet  —  that  is,  the  velocity  of  P  will  be  double  the  velo- 
city of  W ;  and,  therefore,  on  the  principle  ~of  virtual  velocities,  the 
advantage  will  be  2  —  that  is  to  say,  1  Ib.  suspended  at  P  will  sustain  2 
Ibs.  suspended  at  "W. 

42.  Principle  of  Tension.  —  The  single  cord  P  Q  B  D  will  have  the 
same  tension  in  every  part ;  now,  W  hangs  by  the  two  cords  B  D  and  A  Q ; 
therefore  each  cord  must  sustain  a  weight  equal  to  one  half  W  —  that  is, 
4* 


42 


NATURAL    AND    EXPERIMENTAL    PHILO SOPHY. 


the  cord  A  Q  will  have  a  tension  of  one  half  W ;  but  this  tension  is  re- 
sisted by  the  power  at  P  ;  therefore  P  must  also  be  one  half  W. 

In  the  annexed  system  there  are  two  movable  pulleys,  A  and  B,  and 

one  fixed  pulley,  C.     Here  the  string  to  which  H . 

the  power  is  attached  passes  over  the  fixed  pul- 
ley C,  then  round  the  movable  pulley  A,  and 
has  its  extremity  fixed  at  T.  Another  string 
is  attached  to  the  block  of  the  pulley  A,  then 
passes  round  the  movable  pulley  B,  and  has  its 
extremity  fixed  at  N.  Here  P  Q  B,  A  T,  being 
a  continuous  cord,  will  be  stretched  equally 
throughout  the  whole  of  its  length;  and  the 
cords  A  R  and  S  T  will  each  have  a  tension  P 
Ibs. ;  and,  therefore,  a  weight  of  2  P  Ibs.  must 
be  suspended  from  D.  In  like  manner,  since 
D  B  L  N  is  a  continuous  cord,  L  N  and  B  D 
will  have  the  same  tension  —  that  is,  each  of 
them  will  have  a  tension  of  2  P  Ibs.;  and, 
therefore,  a  weight  of  twice  2  P  Ibs.,  or  4  P 
Ibs.,  must  be  suspended  from  K  ;  that  is  to  say, 
in  the  system  represented  in  Fig.  41,  we  have  W=  4  P. 

In  this  system,  (see  Fig.  42,)  a  single  or  continuous 
cord  passes  round  the  wheels  ;  therefore  every  portion 
of  the  cord  must  have  the  same  tension  ;  but  W  hangs 
by  six  cords ;  therefore  each  cord  will  carry  one  sixth 
of  the  weight*  W,  and,  consequently,  the  power  P 
must  also  be  one  sixth  of  W  ;  that  is,  W  =  6  P. 

By  means  of  a  fixed  pulley  (see  Fig.  43)  a  man 
may  raise  himself  to  any  height,  or  let  himself  down 
to  any  depth.  Fire  escapes  have  been  constructed  on 
this  principle. 

43.  Pulleys  are  frequently  employed  for  changing  .„ 


Fig.  41. 


Fig.  43, 


Fig.  44. 


THE    INCLINED    PLANE. 


43 


the  direction  of  motion.  Fig.  44  shows  the  manner  of  converting  a 
horizontal  motion  into  a  vertical  motion.  C  and  B  are  two  fixed  pul- 
leys, having  a  continuous  cord  H  C  B  A  passing  over  them ;  A  is  the 
weight  to  be  raised  by  means  of  a  power  applied  to  the  horizontal  rope 
C  H.  In  this  case  there  is  no  mechanical  advantage  gained. 

This  figure  (45)  represents  a  system  of  pulleys  called 
the  Spanish  barton.  A  and  C  are  two  movable  pul- 
leys, and  B  is  a  fixed  pulley ;  P  A  C  G  H  is  a  contin- 
uous cord  passing  over  the  two  movable  pulleys,  having 
the  power  P  at  one  extremity,  and  the  other  extrem- 
ity fixed  to  a  hook  H ;  A  B  D  E  is  another  continuous 
cord  passing  over  the  fixed  pulley  B  D,  and  connecting 
the  blocks  of  the  two  movable  pulleys  A  and  E.  Let 
P  =  1  lb.,  then  the  cord  P  A  C  G  H,  being  a  single 
cord,  the  portions  P  A,  A  C,  and  G  H  will  each  have 
a  tension  of  1  lb. ;  but  the  cord  A  B  has  a  tension  of  2 
Ibs.,  for  it  sustains  the  tensions  of  A  P  and  A  C.  Now, 
A  B  D  E  being  a  single  cord,  the  cord  E  D  has  the 
same  tension  as  the  cord  A  B ;  that  is,  E  D  must  sus- 
tain a  tension  of  2  Ibs. ;  but  the  cords  G  H  and  A  C 
have  each  a  tension  of  1  lb.  ;  therefore  W  must  be  4 
Ibs.,  in  order  to  produce  the  tensions  of  G  H,  E  D,  and 
C  A.  Hence,  if  P  be  1  lb.,  W  must  be  4  Ibs. 


Fig.  45. 


The  Inclined  Plane. 

44.  When  a  horse  draws  a  load  up  a  hill,  the  road  forms  an  inclined 
plane,  and  the  more  gentle  the  slope  the  more  easily  does  the  horse  draw 
the  load.  The  vertical  space  through  which  the  weight  or  load  is  raised 
is  the  vertical  elevation  of  the  hill ;  but  the  actual  space  over  which  the 
horse  draws  the  load  is  the  inclined  side  of  the  hill ;  therefore  the  advan- 
tage gained  by  the  inclined  plane  will  be  the  number  of  times  that  the 
length  of  the  plane  is  greater  than  its  vertical  height :  thus,  if  the  length 
of  the  inclined  plane  be  double  its 
height,  then  the  advantage  gained 
will  be  2  ;  that  is  to  say,  a  weight 
of  2  cwt.  would  only  require  a 
power  of  1  cwt.  to  draw  it  up  the 
plane,  (supposing  that  there  were 
no  friction,) 

Inclined    planes   are    constantly 
used  for  rolling  casks  into  carts. 

The  inclined  plane,  as  a  mechani-  Fig.  46. 


44 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  47. 


cal  power,  is  also  frequently  used  in  connection  with  frictional  rollers. 
In  this  way  workmen  are  enabled  to  raise  heavy  stones  into  a  cart,  as 
shown  in  Fig.  46.  As  the  rollers  are  disengaged  at  the  lower  end  of  the 
stone  they  are  put  in  at  the  upper  end ;  so  that  three  or  more  rollers  are 
kept  continually  beneath  the  stone  as  it  is  being  rolled  forward. 

Let  A  C  (Fig.  47)  represent  an  incline,  A  B 
its  horizontal  base,  B  C  its  vertical  height,  and 
BAG  its  angle  of  elevation.  Let  W  be  the 
weight  placed  upon  the  plane,  and  P  the  power 
of  drawing  up  this  weight,  by  means  of  the  cord 
P  B  W  passing  over  the  pulley  B  D,  the  cord 
D  W,  in  this  case,  being  parallel  to  the  plane. 

To  find  the  ratio  of  the  vertical  velocities  of 
P  and  "W.  Here,  while  "W  moves  from  A  to  C, 
it  will  have  been  raised  the  vertical  height,  B  C, 
of  the  plane,  and  the  cord  D  W  being  shortened 
a  space  equal  to  A  C,  P  will  have  descended  a 
space  equal  to  A  C,  the  length  of  the  plane; 
hence  the  velocities  of  P  and  W,  estimated  in  a  vertical  direction,  will 
be  to  each  other  as  the  length  of  the  plane  to  its  height ;  therefore  the 
advantage  gamed  will  be  equal  to  the  length  of  the  plane  divided  by  its 
height ;  that  is,  ^=  ^.  If,  for  example,  A  C  =  7  feet,  and  B  C  = 

2  feet,  then  the  advantage  gained  will  be  7  -h  2  =  3£  ;  that  is  to  say,  a 
power  P  of  1  cwt.  will  sustain  a  weight  W  of  3£  cwt. 

The    Wedge. 

45.  This  mechanical  power  is  merely  a  movable  inclined  plane.  It  is 
chiefly  used  in  splitting  timber,  and  in  splitting  rocks  in  quarries.  All 
sharp-edged  tools,  such  as  knives,  axes,  &c.,  act  upon  the  principle  of 
the  wedge.  The  power  of  the  wedge  depends  upon  the  sharpness  of 
its  edge. 

Let  ABC  represent  a  movable  inclined 
plane,  or  wedge,  sliding  along  the  surface  HE, 
by  the  force  of  a  pressure  P  applied  to  the 
back  B  C  of  the  wedge  in  a  direction  parallel 
to  H  B, ;  and  let  "W  be  a  heavy  rod  resting 
upon  the  inclined  side  A  C,  and  constrained  - 
to  move  in  a  vertical  direction.      Here  the  H 
weight  W  acts  vertically,  and  the  power  P 
horizontally.     As  the  wedge  is  being  pushed  forward,  the  rod  D  W  will 
be  raised ;  so  that,  while  the  wedge  has  passed  over  a  space  equal  to  its 


Fig.  48. 


THE    SCREW. 


45 


length  B  A,  the  rod  will  have  been  raised  through  a  space  equal  to  the 
thickness  B  C  of  the  wedge ;  that  is,  while  the  pressure  P  has  passed 
over  a  space  equal  to  A  B,  the  weight  W  has  passed  over  a  space  equal 
to  B  C ;  hence  the  advantage  gained  will  be  equal  to  the  number  of 
times  that  the  thickness  of  the  wedge  is  contained  in  its  length ;  thus, 
if  the  length  A  B  be  9  niches,  and  the  thickness  B  C  1£  inches,  the 
advantage  gained  will  be  9  -z- 1£  =  6  ;  that  is  to  say,  a  pressure  of  1  Ib. 
applied  to  the  head  of  the  wedge  will  produce  an  upward  pressure,  in 
the  direction  D  W,  of  6  Ibs. 

Here  Fig.  49  shows  the  form  of  the  wedge  as  it  is  employed  in  split- 


Fiff.  49. 


Fig.  50. 


ting  timber,  where  C  E  is  the  length,  D  C  the  edge,  and  G  B  or  AF  the 
thickness.  In  Fig.  50,  the  resistance  acting  at  F  arises  from  the  adhe- 
sion of  the  material  that  is  being  split ;  and  the  power  applied  at  A  B  is 
the  impetus  given  by  the  stroke  of  a  heavy  mallet.  The  great  power  of 
the  wedge,  xised  in  this  manner,  depends  almost  entirely  upon  the  ^cork, 
accumulated  in  the  mallet,  being  at  once  delivered  upon  the  head  of  the 
wedge. 

The  wedge  is  frequently  employed  in  raising 
great  weights  for  a  short  distance ;  in  such  cases 
two  wedges  are  made  to  act  together,  as  in  the 
annexed  figure,  where  A  B  d  c  aad  d  b  a  c  rep- 
resent two  similar  wedges  employed  for  raising 
the  mass  W,  by  simultaneous  strokes  given  to 
the  heads  A  c  and  d  b.  It  is  evident  that  the 
plane  of  a  b  will  always  be  parallel  to  A  B. 

The  Screw. 

46.   In  this  simple  machine  the  power  moves  in  a  circle  whose  radius 
is  the  length  of  the  lever,  or  arm  of  the  screw,  whilst  the  weight  or 


Fig.  51. 


46  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

resistance  is  moved  in  a  right  line  having  the  direction  of  the  axis  of 
the  cylinder  on  which  the  threads  of  the  screw  are  formed.  A  screw 
may  be  regarded  as  a  movable  inclined  plane  formed  upon  the  surface  of 
the  cylinder ;  for  if  we  suppose  one  revolution  of  the  thread  to  be  un- 
wrapped, it  will  form  an  inclined  plane,  in  which  the  circumference  of 
the  cylinder  will  be  the  length  of  the  plane,  and  the  distance  between 
the  threads  the  height  of  the  plane. 

Let  cname  (Fig.  52)  be  a  spiral  groove  cut  upon  a  cylinder  after  the 


Fig,  52. 


manner  just  described  ;  C  D  the  axis  upon  which  the  cylinder  turns  ; 
A  B  a  rod  parallel  to  the  axis  C  D,  and  having  a  pin  or  tooth,  c,  fitting 
the  groove  of  the  screw.  Now,  when  the  handle  C  P  is  turned  in  the 
direction  of  the  arrow,  the  pin  c,  with  its  rod  A  B,  .is  moved  towards  the 
right ;  so  that  in  one  revolution  the  pin  will  have  moved  from  c  to  a,  the 
distance  between  the  threads  of  the  screw ;  and  in  the  second  revolution 
it  will  have  moved  from  a  to  e,  and  so  on.  The  rod  A  B  will  thus  be 
moved  in  a  rectilinear  path,  parallel  to  the  axis  CD.  In  one  revolution 
of  the  handle,  therefore,  the  power  P  will  have  passed  over  a  space  equal 
to  the  circumference  of  the  circle  described  by  the  handle,  and  the  weight 
or  resistance  W  will  have  moved  over  a  space  equal  to  the  distance  be- 
tween the  threads  of  the  screw.  Hence  the  advantage  gained  will  be 
equal  to  the  circumference  of  the  circle  described  by  the  power  P  divided 
by  the  distance  between  the  threads  of  the  screw.  Thus,  if  the  circum- 
ference described  by  the  handle  PC  be  20  inches,  and  the  distance  c  a 
between  the  threads  of  the  screw  £  inqh,  then  the  advantage  of  pressure 
gained  will  be  20  -j-  £  =  40  ;  that  is  to  say,  if  a  pressure  of  50  Ibs.  be 
applied  at  P,  it  will  produce  a  pressure  of  40  tunes  50  Ibs.,  or  2000  Ibs., 
in  the  direction  A  B. 

In  the  place  of  a  single  tooth,  c,  and  the  rod,  A  B,  it  is  customary  to 
have  a  series  of  teeth,  in  the  form  of  a  reverse  or  hollow  screw,  exactly 
fitting  the  spiral  groove  formed  on  the  cylinder  or  solid  screw  C  D  ;  the 
reverse  screw  thus  formed  is  called  the  nut.  In  most  applications  of  the 
screw,  the  nut  revolves,  while  the  solid  screw  moves  in  a  longitudinal 
direction. 


WHEEL    WORK. 


47 


The   Common  Press. 

47.  The  screw  is  used  in  cases  where  a  great  pressure  is  to  be  exerted 
through  a  small  space.  The  common  press  is  one  of  the  most  useful 
applications  of  this  mechanical  power. 

Fig.  53  represents  a  bookbinder's  press, 
where  §  S  is  the  solid  screw  working  in 
the  hollow  screw  or  nut  n,  resting  on  the 
fixed  board  c  ;  B  the  press  board,  fixed 
to  the  top  of  the  screw,  and  admits  'of 
being  moved  vertically  between  the  sides 
of  the  frame.  The  solid  screw  S  S  is 
incapable  of  revolving,  but  moves  longi- 
tudinally, or  in  the  direction  of  its  length  ; 
whereas  the  nut  n  revolves,  but  does 
not  move  longitudinally,  or  in  the  direc- 
tion of  the  length  of  the  screw.  The 
nut  is  turned  by  means  of  the  lever  P, 
which  is  inserted  in  the  holes  formed  on 
the  edge  of  the  nut.  The  material  to 
be  compressed  is  placed  between  the 
press  board  B  and  the  fixed  beam  D. 

In  one  turn  of  the  lever  P,  the  screw 
S  S,  with  its  press  board  B,  is  moved 

upward  a  space  equal  to  the  distance  between  the  threads  of  the  screw. 
Hence  we  have  the 

space  described  by  P 
Advantage  gained 


Fig.  53. 


the  thread  s 

Thus,  if  P  sweep  a  circle  of  20  feet,  or  240  inches,  and  the  distance 
between  the  threads  of  the  screw  be  |  of  an  inch,  then  the  advantage 
of  pressure  gained  will  be  240  -r  |  =  320  ;  that  is,  if  a  man  exert  a 
pressure  of  56  Ibs.  upon  the  extremity  of  the  lever,  then  the  upward 
pressure  produced  upon  the  press  board  will  be  320  times  56  Ibs.,  or 
17,920  Ibs.  =  8  tons. 


Wheel   Work. 

48.  Motion  may  be  communicated  from  one  axis  to  another  by 
means  of  cords  or  straps,  as  in  case  shown  in  Pig.  34,  or  by  means  of 
toothed  wheels,  as  shown  in  Fig.  35.  If  the  toothed  wheel  B  drive 
the  toothed  wheel  A,  (Fig.  54,)  then  B  is  called  the  driver,  and  A  the 
follower.  Wheels  acting  in  this  manner  are  also  called  spur  wheels. 
Small  toothed  wheels  are  called  pinions ;  thus  B  may  be  called  a  pinion 


48 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


in  relation  to  A.     Two  toothed  wheels  are  said  to  be  in  gear  wrhen  their 
teeth  are  engaged  together,  and  out  of  gear  when  they  are  separated. 

If  B  contain  15  teeth,  and  A  90,  then  B  must  turn  round  six  times 
in  order  that  A  may  turn  round  once.  Or,  generally,  if  A  make  one 
revolution,  the  number  that  B  will  make  is 
found  by  dividing  the  number  of  teeth  in  A 
by  the  number  in  B.  Or,  since  the  number 
of  teeth  in  the  wheels  is  proportional  to  their 
radii,  the  number  of  revolutions  of  B  will  also 
be  found  by  dividing  the  radius  of  A  by  the 
radius  of  B ;  thus,  let  the  radius  of  A  be  15 
in.,  and  that  of  B  3  in.,  then  B  will  make 
five  revolutions  while  A  makes  one. 

In  the  train  of  wheels  represented  in  Fig.  55, 
the  motion  of  the  axis  Nx  S  is  transmitted  to 
three  distinct  parallel  axes.     N,  is  the  first 
driving  wheel,  m  its  follower ;  N2  is  the  second  driving  wheel,  n2  its  fol- 
lower ;  and  so  on.     Let  the  number  of  teeth  in  Nx  =  36,  in  n^  =  9, 


Fig.  55. 

in  N2  =  32,  in  w2  =  8,  in  N3  =  35,  and  in  ns  =  7  ;  then,  while  the  axis 
of  N!  makes  one  revolution,  the  axis  of  «3  will  make  80.  In  order  to 
prove  this,  suppose  the  driver  Nx  to  make  one  revolution,  then,  while  Nx 
makes  one  revolution,  the  number  of  revolutions  which  9t1  will  make 
._  36  _:_  9  —  4.  Now,  as  N2  revolves  on  the  same  axis  as  nv  the  driver 
N2  will  make  four  revolutions  while  N  makes  one.  In  like  manner,  N3 
will  make  four  revolutions  while  N2  makes  one ;  but  N2  makes  four 
revolutions  while  Na  makes  one ;  therefore  N3  must  make  four  times 
four  revolutions,  or  sixteen  revolutions.  In  like  manner,  n3  will  make 
five  revolutions  while  N3  makes  one  ;  but  N3  makes  sixteen  revolutions 
while  1^  makes  one ;  therefore  ns  will  make  sixteen  times  five  revolu- 
tions, or  eighty  revolutions,  while  Na  makes  one. 

When  motion  is  to  be  transferred  from  one  axis  to  another  axis  at 
right  angles  to  it,  we  must  use  crown  wheels,  bevelled  wheels,  or  face 
wheels. 


WHEEL    WORK. 


Crown    Wheels. 

49.  This  figure  56  represents  a  crowi 
wheel  B,  with  its  pinion  A,  having  then- 
axes  at  right  angles  to  each  other.  The 
teeth  in  the  crown  wheel  are  cut  on  the 
edge  of  a  hoop,  and  the  pinion  is  made 
thicker  than  usual.  This  kind  of  wheel 
is  used  in  clock  and  watch  work. 


Fig.  56. 
Face   Wheel  and  Lantern. 

50.  In  Fig.  57,  F  represents  a  face  tcheel,  with  its  lantern  L.  Motion 
is  here  transferred  from  a  vertical  axis  to  a  horizontal  one.  The  teeth 
inserted  into  the  face  of  the  wheel  F  are  called  cogs>  which  are  now 
usually  made  of  iron,  while  the  round  staves  forming  the  teeth  of  the 
lantern  are  made  of  hard  wood ;  for  it  has  been  ascertained  that  iron 


Fig.  57. 

cogs  work  with  less  noise  and  friction  upon  wooden  staves,  than  when 
the  cogs  and  staves  are  made  of  the  same  material.  The  face  wheel 
and  lantern  have  been  much  used  in  mill  work. 

Bevel   Wheels,  or  Bevel  Gear. 

51.  Let  E  B  and  F  B  (Fig.  58)  be 
two  axes  of  rotation,  cutting  each  other  in 
B.  Two  right  cones  ABC  and  B  D  G, 
touching  each  other  in  the  line  B  G  C,  are 
formed  upon  these  axes.  If  the  cone  B  D  C 
revolve  on  its  axis  E  B,  it  will  communi- 
cate, by  rolling  contact,  a  rotatory  motion 
to  the  cone  ABC,  upon  its  axis  F  B. 
5 


Fig.  58. 


50 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


In  practice,  frustra  of  the  cones  are  employed,  as  A  C  G  J,  and 
CDHG. 

These  cones,  or  frustra  of  cones,  will  obviously  perform  their  revolu- 
tions in  the  same  manner  as  the  spur  wheels  in  Fig.  54. 

On  these  smooth  surfaces  of  the  frustra  a  series  of  equidistant  teeth 
may  be  cut,  directed  to  the  apex  B  of  the  cone,  so  that  a  line  passing 
from  the  apex  B  to  the  outline  of  the  teeth  upon  the  bases  of  the  cones 


Fig.  59. 


shall  touch  the  teeth  in  every  part;  as  shown  in  the  annexed  cut, 
(Fig.  59,)  where  B  is  the  apex  of  the  cones  B  A  C  and  B  D  C,  f  and  E 
the  two  axes  of  the  bevel  wheels  A  C  and  D  C,  intersecting  in  the 
apex  B. 

Wheels  cut  in  this  manner  are  called  bevel  gear.  Two  bevel  wheels 
of  this  kind  will  always  communicate  motion  from  one  axis  to  another, 
provided  these  axes  intersect  each  other;  this  point  of  intersection  is 
always  made  the  apex  of  the  frustra  forming  the  bevel  wheels. 

Rack  and  Pinion. 

52.  When  a  circular  motion  is -to  be  changed  into  a  rectilinear  one, 
the  teeth  are  cut  upon  the  edge  of  a  straight 
bar,  A  B,  (Fig.  60,)  so  that  they  may  work 
with  the  teeth  upon  the  wheel  or  pinion  P. 
The  toothed  bar  A  B  is  called  a  rack,  and  is 
constrained  to  move  in  its  rectilinear  path  by 
guides  or  rollers.  Fig.  60. 


WHEEL    WORK. 


51 


Fig.  61. 


53.  The  way  in  which  a  continuous  motion  is  given  to  a  wheel  by 
means  of  a  treadle  board  is  shown  in  Fig.  61.     c  d  is  a  treadle  board, 
or  a  board  that  is  moved  by  the  pressure  of  the  foot ;  the  cord  c  a  e 
passes  over  the  pulley  a,  and  is  attached  to 

the  crank  m  e  of  the  wheel  m.  While  the 
extremity  c  of  the  treadle  describes  a  recip- 
rocating circular  motion,  the  wheel  m  re- 
volves continuously. 

54.  Fig.  62  shows  the  way  in  which  a 
forge  hammer  is  moved  by  the  continuous 
circular  motion  of  a  drum  wheel  or  cylinder. 
The  cylinder  a  (see  Fig.  62)  has  five  pecu- 
liar shaped  teeth  upon  it,  called  wipers  or 
tappets,  which  strike  the  extremity  of  the 

handle  of  the  hammer  at  successive  intervals.     The  hammer  b  turns 
upon  a  lever  b  e,  whose  axis  is  C ;  the  extremity  e  of  the  leyer  is  de- 
pressed by  the  wipers,  and  thus  the  hammer 
is  raised ;  but  the  moment  the  wiper  disen- 
gages itself  from  the  lever,  the  hammer  falls 
by  its  weight,  and  strikes  the  heated  iron 
placed  upon  the  anvil  A.     In  this  case,  the 
hammer  would  make  five    lifts   and  five 
strokes  for  every  revolution  of  the  wheel. 

In  this  mechanism,  a  continuous  circular 
motion  is  converted  into  an  intermittent  re- 
ciprocating, or  up  and  down  motion. 

55.  Fig.  63  shows  the  way  in  which  a  continuous  circular  motion 
may  be  converted  into  a  continuous  reciprocating,  or  back  and  forward 


Fig.  62. 


o 


Fig.  63. 

motion,  c  is  a  wheel  partially  furnished  with  teeth,  acting  on  two 
racks,  e  and  n,  placed  on  different  sides  of  it ;  the  teeth  in  these  racks 
are  alternately  engaged  by  the  teeth  of  the  wheel,  so  that  the  continuous 
circular  motion  of  the  wheel  c  gives  a  regular  back  and  forward  motion 
to  the  rod  A  B,  placed  between  frictional  rollers. 


52     NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 

EXERCISES  ON  MECHANICS. 

1.  A  body  moves  through  a  space  of  540  feet;  its  velocity  is  6  feet 
per  second  :  what  will  be  the  time  of  its  motion  ?          Am.  l£  hours. 

2.  A  carrier  pigeon,  flying  with  a  uniform  speed  of  15  feet  per  sec- 
ond, was  24  hours  in  passing  from  a  ship  at  sea  to  the  land  :  required 
the  distance  in  miles.  Ans.  245/1  miles. 

3.  A  ball  of  7  Ibs.  is  moving  with  a  velocity  of  9  feet  per  second ; 
and  a  ball  of  3  Ibs.  moves  with  a  velocity  of  14  feet  per  second :  what 
are  their  comparative  momenta?  Ans.  3  to  2. 

4.  A  falling  body  required  7  seconds  to  reach  the  ground :  through 
what  space  did  it  fall?  Ans.  788T^  feet. 

5.  One  arm  of  a  lever  is  44  feet  long,  and  the  other  is  5  feet :  what 
power  applied  to  the  longer  arm  will  balance  500  Ibs.  at  the  shorter  arm  ? 

Ans.  56  Ibs.,  13^  oz. 

6.  At  what  distance  from  the  fulcrum  of  a  lever  of  the  second  kind 
that  is  20  feet  long,  must  a  weight  of  112  Ibs.  be  placed,  so  that  it  may 
be  sustained  by  a  power  of  50  Ibs.  ?  Ans.  8  feet,  11-f  inches. 

7.  A  wheel  of  10  feet  diameter,  with  a  power  of  5  Ibs.,  balances  a 
weight  of  150  Ibs. ;  what  is  the  radius  of  the  axle  ?        Ans.  2  inches. 

THE  STEAM  ENGINE. 

DIFFERENT   PIECES    OF   MECHANISM    CONNECTED   WITH   THE 
STEAM   ENGINE. 

1.  THEBE  are  a  variety  of  interesting  pieces  of  mechanism  connected 
with  the  steam  engine,  which  merit  a  minute  description. 

The  Crank  and  Fly  Wheel 

2.  The  crank  and  connecting  rod  are  used  for  converting  the  recip- 
rocating motion  of  the  extremity  of  the  great  beam  of  the  steam  engine 
into  a  continuous  circular  motion.     When  we  turn  a  grindstone,  we 
employ  the  peculiar  motion  of  the  crank  and  connecting  rod,  where 
the  handle  of  the  grindstone  serves  the  purpose  of  the  crank,  and  the 
arm  that  of  the  connecting  rod.     The  crank,  with  its  connecting  rod 
and  fly  wheel,  is  represented  in  Fig.  64.     C  is  an  axis,  to  which  the 
fly  wheel  F,  or  any  other  wheel  work,  may  be  attached  ;  C  D  is  a  link 
or  arm,  called  the  crank,  fixed  to  the  axis  C,  and  having  a  joint  at  D,  to 
which  the  connecting  rod  D  A  is  attached.     Now,  if  an  up  and  down 
motion  be  given  to  D  A,  the  extremity  D  of  the  crank  will  move  in  a 
circle,  and  thus  a  continuous  rotation  will  be  given  to  the   axis   C 


THE    STEAM    ENGINE. 


53 


» It  t: 


When  the  crank  arrives  at  the  position  C  n,  where  it  is  in  the  same  line 
with  the  connecting  rod,  it  is  said  to  be  in  one  of  its  dead  points,  for 


Fig.  64. 


then  the  pressure  upon  the  connecting  rod  has  no  effect  in  turning  the 
crank ;  but  in  general,  the  inertia  of  the  machinery  and  fly  wheel  F 
carries  the  crank  beyond  the  dead  points.  It  will  be  seen  that  the 
crank  has  to  pass  over  two  dead  points  in  the  course  of  one  revolution. 
In  order  to  avoid  this  irregularity  in  the  action  of  the  connecting  rod, 
two  cranks  are  sometimes  placed  on  the  same  axis,  at  right  angles  to 
each  other.  The  connecting  rod  in  a  steam  engine  is  usually  attached 
to  the  extremity  E  of  the  great  beam. 

The  fly  wheel  is  not  only  a  regulator  of  motion,  but  it  is  also  an  ac- 
cumulator of  motion.  It  simply  consists  of  a  large,  heavy  wheel,  to 
which  motion  is  usually  given  by  a  crank ;  thus,  in  Fig.  64,  F  is  the 
fly  wheel,  revolving  upon  the  axis  C. 

The  fly  wheel  may  be  regarded  as  a  reservoir  of  motion,  in  which  the 
redundant  motion  of  the  machinery  is  accumulated  when  the  work  to  be 
performed  is  less  than  the  work  applied  by  the  moving  power,  and  from 
which  the  machinery  derives  motion  when  the  work  to  oe  performed  is 
greater  than  the  work  applied ;  so  that,  however  variable  the  work  to  be 
performed  may  be,  the  motion  of  the  machinery  is  always  maintained 
pretty  nearly  uniform. 
5* 


54  NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

•* 

The   Sun-and-planet    Wheel. 

3.  This  beautiful  contrivance  was  employed  by  Watt  as  a  substitute 
for  the  crank.  It  consists  of  two  toothed  wheels,  one  of  which  revolves 
round  the  circumference  of  the  other,  somewhat  similar  to  the  manner 
in  which  a  planet  and  its  satellite  revolve  round  the  sun ;  hence  the  name 
given  to  this  mechanical  combination. 

The  toothed  wheel  B  (Fig.  65)  is  fixed  to  the  extremity  of  the  con- 
necting rod  C  D,  so  as  not  to  be  allowed  to  turn  on  its  centre ;  A  is 
another  toothed  wheel,  fixed  to  the  axis  e  of  the  fly  wheel  D  ;  a  link 
connects  the  centres  of  the  two  wheels  A  and  B,  and  serves  to  keep  them 
in  gear.  Now,  when  the  great  beam  has  made  an  up  and  down  stroke, 
the  link  e  o,  connecting  the  centres  of  the  two  toothed  wheels,  will  have 
performed  a  complete  revolution  round  the  centre  e,  exactly  as  a  common 
crank  would  do ;  but  as  the  two  wheels  A  and  B  are  fixed  to  their  re- 
spective centres,  every  portion  in  the  circumference  of  B  will  have  been 
brought  in  contact  with  the  wheel  A,  which  thus  receives  a  continuous 


Fig.  65. 


circular  motion.  Assuming  the  wheels  A  and  B  to  be  equal,  then,  while 
the  connecting  rod  makes  an  up  and  down  stroke,  or,  what  is  the  same 
thing,  while  the  wheel  B  makes  one  revolution  round  the  centre  e,  the 
wheel  A,  with  the  fly  wheel  D,  will  have  performed  two  revolutions  ;  for 
in  this  case  every  tooth  in  A  will  have  come  twice  into  contact  with  the 

teeth  on  B. 

• 

Watt's  Parallel  Motion. 

4.  This  beautiful  mechanical  contrivance  is  used  to  convert  the  recip- 
rocating circular  motion  of  the  extremity  of  the  great  beam  of  an  engine 


THE    STEAM    ENGINE. 


55 


into  the  reciprocating  rectilinear  motion  of  the  piston  rod.  It  consists  of 
a  frame  of  link  work  somewhat  in  the  form  of  a  parallel  ruler. 

The  leading  feature  of  the  contrivance  is  represented  in  Fig.  66. 

Let  A  B  and  C  D  be  two  equal  rods,  connected  by  the  link  D  B, 
moving  on  their  respective  fixed  centres  of  motion  A  and  C.  Let  E  be 


Fig.  66. 

the  middle  point  of  the  connecting  link  D  B.  Now,  let  the  rods  be 
moved  to  another  position,  and  let  C  d  e  b  A  be  that  new  position  of  the 
rods  ;  then  the  middle  point  E  or  e  of  the  link  will  have  nearly  moved 
in  a  vertical  right  line.  For  while,  by  this  motion,  the  extremity  B  of 
the  link  is  carried  to  the  left,  the  extremity  D  is  carried  to  the  right, 
and  vice  versa ;  so  that  the  middle  point  E  of  the  link  thus  nearly  moves 
in  a  vertical  line. 

Let  A  K  (Fig.  67)  represent  one  half  of  the  great  beam,  turning  on 
the  centre  A ;  K  B  D  R  link  work  in  the  form  of  a  parallelogram,  hav- 
ing B  K  equal  to.  A  B  ;  C  D  a  rod,  called  the  radius  rod,  turning  on  the 
fixed  centre  C.  Now,  the  rods  A  B  D  C  will  move  in  precisely  the  same 
manner  as  in  the  preceding  figure,  and 
therefore  the  point  E,  in  the  middle 
of  the  link  D  B,  will  very  nearly  de- 
scribe a  vertical  line.  But  since  the 
triangles  ARK  and  A  E  B  are  sim- 
ilar, and  as  A  K  is  the  double  of  A  B, 
the  line  A  R  will  be  the  double  of 
A  E  ;  that  is,  the  point  R  will  always 
be  at  double  the  distance  from  A  that 


Fig.  67. 


56  NATURAL    AND    EXPEKIMENTAL    PHILOSOPHY. 

the  point  E  is  ;  and  therefore  the  path  described  by  E,  will  be  the  same 
as  the  path  described  by  E ;  therefore,  if  the  point  E  moves  in  a  vertical 
line,  the  point  R,  will  also  move  in  a  vertical  line.  The  piston  rod  is 
attached  to  the  point  R,  and  the  piston  rod  Df  the  air  pump  to  the  point 
E  ;  so  that  both  of  these  rods  will  be  moved  in  a  vertical  line. 

The  Eccentric    Wheel. 

5.  A  wheel  is  said  to  be  eccentric  when  it  turns  on  an  axis  which 
does  not  lie  in  the  centre  of  the  wheel.  This  important  piece  of 
mechanism  is  usually  employed  to  give  motion  to  the  slide  valve  of 
the  steam  engine  where  the  axis  of  the  fly  wheel  is  always  the  centre 
of  motion  of  the  eccentric  wheel.  Here  A  is  the  axis  of  the  eccen- 
tric wheel,  C  being  the  centre  of  the  circle ;  a  hoop  J  K  embraces  the 
eccentric  wheel,  so  as  to  allow  the  wheel  to  revolve  freely  within  the 


Fig.  68. 


hoop  ;  a  frame  D  F  E  connects  this  hoop  with  the  extremity  F  of  the 
bent  lever  H  G  F  turning  on  the  fixed  centre  G.  Now,  when  the 
eccentric  wheel  turns  in  the  direction  of  the  arrow  of  the  figure,  the 
frame  E  D  F  is  pushed  to  the  right,  and  the  pin  F  describes  an  arc  of  a 
circle  in  the  same  direction,  on  G  as  a  centre  ;  when  the  lob  side  of  the 
eccentric  has  passed  the  line  of  the  centres  of  motion  A  and  F,  the  frame 
with  the  pin  F  is  then  drawn  to  the  left,  and  so  on  ;  so  that  the  con- 
tinuous circular  motion  of  the  eccentric  wheel  produces  a  reciprocating 
circular  motion  in  the  pin  F.  This  motion  of  F  gives  a  reciprocating 
motion  to  the  rod  H  I,  to  which  is  attached  the  slide  valve  of  the  engine. 

The   Governor. 

6.  This  is  one  of  the  most  important  regulators  of  machinery.  When 
the  speed  of  the  machinery  is  too  great,  this  contrivance  checks  the  sup- 
ply of  the  moving  force ;  and,  on  the  contrary,  when  the  speed  is  too 
slow,  it  increases  that  supply.  This  simple  and  beautiful  piece  of  mech- 
anism (see  Fig.  69)  consists  of  two  heavy  balls  E  E,  attached  to  the 
extremities  of  the  rods  /  c  E,  which  pass  through  a  slit  in  the  vertical 


THE    STEAM   ENGINE. 


57 


axis  D  D,  and  turn  on  the  centre  e,  opening  and  closing  like  a  pair  of 
shears.  The  links/  h,  having  joints  at  /  and  h,  connect  the  two  rods/  e 
E  with  a  ring  h  h  D,  which  slides  freely  upon  the  vertical  axis  D  D,  to 
which  a  rotary  motion  is  given  by  means  of  a  belt  passing  round  the 


Fig.  69. 

pulley  d.  The  lever  F  G  H,  turning  on  the  centre  Gr,  is  connected  with 
the  sliding  piece  or  ring  A  A  D  at  the  extremity  F,  and  has  a  link  H  w 
attached  to  the  extremity  H.  The  link  H  w  turns  the  axis  of  the  throttle 
valve  Z,  which  opens  and  closes  the  port  of  the  steam  pipe  A  A  a,  pro- 
ceeding from  the  boiler  to  the  cylinder.  Now,  when  the  spindle  D  D 
revolves  with  an  increasing  velocity,  the  balls  E  E  fly  out  from  the  centre 
of  motion,  (by  the  centrifugal  force  thus  generated  ;)  the  sliding  piece 
or  ring  h  AD,  with  the  extremity  F  of  the  lever,  is  drawn  downwards, 


58 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


while  the  extremity  H  is  raised,  and  the  axis  of  the  throttle  valve  Z  is 
turned  round,  so  as  to  close  the  opening  of  the  steam  pipe,  thereby  re- 
ducing the  supply  of  steam.  The  contrary  effect  is  produced  when  the 
velocity  of  the  spindle  D  D  is  decreasing  ;  that  is,  the  balls  fall  towards 
the  axis  D  D,  and  the  throttle  valve  Z  is  turned,  so  as  to  increase  the 
supply  of  steam.  Hence  it  appears  that  when  the  speed  of  the  engine 
passes  beyond  a  certain  limit,  the  throttle  valve  tends  to  check  the  supply 
of  the  steam,  or  moving  principle ;  while,  on  the  contrary,  when  the 
speed  of  the  engine  is  less  than  this  mean  limit,  the  throttle  valve  is 
opened,  so  as  to  allow  a  greater  quantity  of  steam  to  pass  through  the 
steam  pipe. 

THE    STEAM    BOILER    AND    ITS    APPENDAGES. 

7.  The  steam  boiler  is  made  of  thick  sheet  iron  or  copper  plates, 
riveted  strongly  together  to  resist  the  expansive  pressure  of  the  steam 
as  well  as  the  destructive  action  of  the  great  heat  which  is  applied  to 
them.  Steam  boilers  are  made  of  various  forms.  Fig.  70  represents  a 


Fig.  70. 

longitudinal  as  well  as  a  cross  section  of  what  is  called  the  butterly 
boiler,  which  is  much  used  in  our  manufacturing  districts  ;  A  represents 
the  ash  pit,  F  F  the  furnace,  B  the  boiler,  and  H  H  the  level  of  the 
water  in  the  boiler.  The  concave  form  given  to  the  bottom  of  the  boiler 
obviously  brings  a  larger  extent  of  surface  in  contact  with  the  flame  than 
would  take  place  if  the  bottom  were  flat. 

The  steam  boiler  has  various  appendages,  which  require  special  notice. 

The  Safety  Valve. 

8.  The  safety  valve  is  used  to  secure  the  boiler  from  bursting  by  the 
excessive  pressure  of  the  steam.  Fig.  71  represents  the  lever  safety  valve, 
where  A  B  is  the  lever  with  its  load  L, 
pressing  upon  the  head  of  the  valve  V, 
which  closes  the  opening  S  leading  into 
the  boiler.  By  sliding  the  load  L  along 
the  lever,  any  pressure  may  be  put  upon  Fig.  71. 


THE    STEAM    ENGINE. 


59 


the  valve  that  may  be  found  necessary  to  work  the  engine.  The  divis- 
ions upon  the  lever  enable  the  engineer  to  determine  the  elasticity  of  the 
steam  in  the  boiler. 

The  Steam   Gauge. 

9.  This  instrument  is  designed  to  indicate  the  degree  of  pressure  of 
the  steam  which  is  used  in  working  the  engine.  Fig.  72  represents  a 
mercurial  steam  gauge ;  A  C  D  E  is  a  bent  tube,  open  at  both  extrem- 
ities, passing  from  the  vessel  B  containing  the  steam;  G  is  a  grad- 
uated scale  for  indicating  the  height  of  the  mercury  in  the  leg  D  E. 
When  the  pressure  of  the  steam  is  equal  to  that  of  the  external  air,  the 
mercury  in  the  two  legs  C  D  and  D  E  stands  at  the  same  level,  H  R ; 
but  when  the  pressure  of  the  steam  is  greater  than  the  external  air,  the 
mercury  is  depressed  in  the  leg  C  D  and  elevated  in  the  leg  D  E.  The 
excess  of  pressure  of  the  steam  above  that  of  the  atmosphere  is  found  by 
observing  the  difference  of  the  levels  of  the  mercury  in  the  legs  D  E  and 
D  C,  and  then  allowing  half  a  pound  as  the  pressure  of  the  steam  on 


each  square  inch  for  every  inch  in  the  difference  of  the  levels.  The  bent 
tube  is  frequently  made  of  iron.  In  this  case  a  float  F,  with  a  rod  and 
pointer,  is  inserted  into  the  open  end  of  the  tube.  As  the  float  F  is  raised 
or  depressed  with  the  mercury,  the  pointer  is  made  to  indicate  the  differ- 
ence of  the  levels  of  the  mercury  in  the  two  legs  of  the  instrument. 


60 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


The    Water   Gauge. 

10.  This  simply  consists  of  a  bent  glass  tube  A  D  C  B, 
(Fig.  73,)  where  one  extremity  A  enters  the  boiler  above 
the  proper  level  H  R  of  the  water,  and  the  other  extrem- 
ity B  enters  below  the  proper  level.  As  the  water  must 
stand  at  the  same  level  in  the  glass  tube  D  C  that  it  does 
in  the  boiler,  the  eye  of  the  engineer  will  at  once  see  what 
depth  of  water  is  in  the  boiler.  Another  kind  of  water 
gauge  is  explained  in  the  general  description  of  the  steam 
engine. 


Fig.  73. 


The   Water  Regulator. 

11.  In*  the  steam  engine  it  is  especially  necessary  that  the  water  in 
the  boiler  should  be  constantly  kept  at  the  same  level,  so  that  as  the 
water  is  being  evaporated  in  the  boiler  fresh  water  should  at  the  same 
time  be  admitted  to  supply  the  waste  thus  created.     Fig.  74  represents  a 
portion  of  the  boiler  A,  with  A  B  a  pipe 

proceeding  from  the  cistern  B  to  supply  the 

boiler  with  water  as  it  may  be  required ;  F 

is  a  stone  float  suspended  by  the  rod  F  C 

passing  through  the  stuffing  box  S ;  this  rod 

is  attached  to  the  extremity  C  of  the  lever 

C  F  turning  upon  the  fulcrum  or  centre  D  . 

V  is  a  valve  opening  and  closing  the  top  of 

the  pipe  A  B,  and  attached  to  the  point  E 

of  the  lever  C  F  ;  F  is  a  counterpoise  which 

aids  in  depressing  the  valve  V.     Now,  when 

the  water  in  the  boiler  descends  below  its 

proper  level,  the  float  F  also  descends,  and 

by  depressing  the  extremity  C  of  the  lever 

elevates  the  valve  V,  and  thus  allows  the 

water  to  flow  into  the  boiler  as  required.     On  the  contrary,  as  the  water 

rises  in  the  boiler  the  float  F  also  rises,  and  by  elevating  the  extremity  C 

of  the  lever  depresses  the  valve  V,  and  thus  stops  the  flow  of  water  into 

the  boiler ;  thus  a  certain  mean  quantity  of  water  is  always  maintained 

in  the  boiler. 

The  Self-regulating  Damper. 

12.  The  rate  at  which  steam  is  generated  in  the  boiler  should  be  equal 
to  the  rate  at  which  it  is  consumed  in  the  cylinder ;  or,  what  is  the  same 
thing,  the  steam  in  the  boiler  should  be  maintained  at  a  constant  pres- 
sure.   In  order  to  effect  this,  some  connection  must  be  formed  between 


Fig.  74. 


THE   STEAM   ENGINE. 


61 


the  pressure  of  the  steam  in  the  boiler  and  the  heat  of  the  furnace,  since 
the  pressure  of  the  one  depends  upon  the  heat  of  the  other.  This  has 
been  accomplished  by  the  following  contrivance  :  Fig.  75,  B  A  is  a  tube 
descending  nearly  to  the  bottom  of  the  boiler  A ;  F  is  a  float  suspended 
by  a  chain  P  Q  D  passing  over  the  pulleys 
P  and  Q;  D  is  a  damper  acting  as  a 
counterpoise  to  the  float,  and  opening  or 
closing,  as  the  case  may  be,  the  mouth  of 
the  flue  L,  and  thereby  increasing  or  de- 
creasing the  draught  of  air  through  the 
fire  K.  Now,  the  level  F  of  the  water  in 
the  tube  A  B  depends  upon  the  pressure 
of  the  steam  in  the  boiler  A ;  but  the  float 
F  rises  and  falls  with  the  water  in  the  tube 
A  B,  and  as  the  float  rises  the  damper  D 
descends,  and  vice  versa;  so  that,  when 
the  pressure  of  the  steam  in  the  boiler  ex- 
ceeds its  proper  limit,  the  water  in  the 
tube  A  B,  together  with  the  float  F, 


Fig.  75. 


ascends,  and  then  the  damper  D  descends  and  closes  the  mouth  of  the 
flue,  thereby  reducing  the  intensity  of  the  heat  of  the  furnace,  and  check- 
ing the  further  generation  of  steam.  On  the  contrary,  when  the  pres- 
sure of  the  steam  falls  below  its  proper  limit,  the  water  in  the  tube,  with 
the  float,  descends,  the  damper  D  is  raised,  and  an  increase  of  draught 
is  given  tp  the  furnace,  which  produces  a  more  rapid  generation  of  steam, 
and  consequently  with  an  increase  to  its  pressure. 


DIFFERENT    FORMS    OF    THE    STEAM    ENGINE. 


Hiero  Engine. 

1.  The  first  steam  engine  was  invented  by 
Hiero  of  Alexandria,  120  B.  C.  It  now  forms 
one  of  our  prettiest  philosophical  toys.  This 
engine  is  represented  in  Fig.  76  :  A  is  a  hol- 
low globe  containing  water,  turning  on  a  ver- 
tical axis  a;  a  b,  &c.,  are  four  horizontal 
tubes  having  their  exterior  orifice  bent  in  the 
same  direction  as  in  Barker's  mill.  When, 
the  water  boils,  steam  issues  from  these  ori- 
fices, and  causes  the  globe  to  rotate  upon  its 
axis. 


Fig.  76. 


62 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  77. 


Savemfs  Engine. 

2.  This  engine  was  used  for  raising  water  from  deep  mines.     The 
principle  on  which  it  acts  may  be  explained  as  follows :  Fig.  77,  C  is  a 
large  cylindrical  vessel,  called  the  receiver,  into  which  steam  enters  by 
the  steam  pipe  S,  communicating  with  a  strong  boiler,  called  the  steam 
boiler,  where  steam  at  a  high  pressure  is  generat- 
ed ;  the  steam  pipe  S  has  a  cock  a,  called  the  steam 

cock,  which  opens  and  closes  the  communication 
of  the  receiver  with  the  steam  boiler  ;  I  is  the  in- 
jection pipe,  which  conveys  a  jet  of  cold  water 
into  the  interior  of  the  receiver  for  the  purpose  of 
condensing  the  steam ;  this  pipe  has  also  a  cock  b, 
called  the  injection  cock ;  these  two  cocks  a  and  b 
are  turned  by  the  same  handle  A,  so  that  when  b 
is  open  a  is  closed,  and  vice  versa  ;  F  is  a  pipe  de- 
scending into  the  water  which  is  to  be  raised ;  at» 
the  top  of  this  pipe  is  the  valve  V,  lifting  up- 
wards ;  E  D  is  a  pipe  proceeding  from  the  bottom 
of  the  receiver  to  the  cistern,  into  which  the  water 
is  to  be  discharged ;  in  this  pipe  is  placed  the  valve  v,  lifting  upwards. 

To  work  the  engine,  the  steam  cock  a  is  opened  and  b  is  shut ;  then 
the  steam,  rushing  along  the  steam  pipe  S,  enters  the  receiver  C,  and 
drives  the  air  out  of  it  through  the  valve  v.  When  the  receiver  is  filled 
with  steam,  the  steam  cock  a  is  closed,  and  at  the  same  time  the  injec- 
tion cock  b  is  opened ;  then  the  jet  of  cold  water  proceeding  from  the 
injection  pipe  instantly  condenses  the  steam  in  the  receiver,  and  a  vacuum 
is  formed.  The  pressure  of  the  atmosphere  on  the  surface  of  the  water 
in  the  well  or  pit  forces  the  water  up  the  pipe  F,  and  nearly  fills  the 
receiver.  The  engineer  now  lays  hold  of  the  handle  A  and  opens  £he 
steam  cock  a,  at  the  same  time  that  he  closes  the  injection  cock  b.  The 
steam  again  enters  the  receiver,  and  by  its  great  elastic  pressure  exerted 
upon  the  surface  of  the  water  forces  the  water  through  the  valve  v,  up 
the  pipe  E  D,  to  the  top  of  the  pit  or  mine.  In  the  same  manner  the 
engine  is  made  to  perform  any  number  of  strokes. 

The  defects  of  this  engine  are  as  follows  :  1.  It  is  limited  in  its  appli- 
cation to  the  raising  of  water ;  2.  There  is  a  great  loss  of  power  at  each 
successive  lift,  occasioned  by  the  steam  coming  in  contact  with  the  cold 
water  in  the  receiver. 

Newcomeris  Engine  with  the  Crank  and  Fly  Wheel. 

3.  This  engine  wae  a  great  improvement  upon  Savery's.     Its  leading 
features  are  represented  in  Fig.  78.     C  is  the  boiler,  communicating  with 
the  cylinder  E  by  means  of  the  steam  pipe  S ;  P  is  the  piston  rod,  con- 


THE    STEAM    ENGINE. 


63 


Fig.  78. 

nected  with  a  solid  piston,  which  works  steam  tight  in  this  cylinder ;  the 
rod  P  of  the  piston  is  connected  with  the  chain  which  coils  round  the 
arched  head  a  6  of  the  great  beam  L  L  ;  so  that  as  the  piston  descends 
the  extremity  of  the  great  beam  is  drawn  down,  and  at  the  same  time 
the  piston  rod  does  not  deviate  from  its  vertical  position  ;  G  is  a  cistern 
of  cold  water  called  the  injection  cistern  ;  from  this  descends  the  injec- 
tion pipe  G  I  K,  (see  also  Fig.  79,)  which 
enters  the  bottom  of  the  cylinder :  K  is  the 
injection  cock;  at  the  opposite  side  of  the 
cylinder  there  is  a  lateral  pipe,  turning  up- 
wards at  the  extremity,  having  a  valve  N, 
called  the  snifting  valve,  lifting  upwards :  Q 
is  the  eduction  pipe,  for  drawing  off  the  water 
formed  in  the  cylinder  ;  the  extremity  D  of 
this  pipe  is  inserted  in  a  vessel  of  water,  and  has 
its  orifice  closed  by  a  valve  lifting  outwards. 
When  the  engine  is  required  to  be  put  in 
action,  —  let  us  suppose  that  the  piston  P  is 
drawn  to  the  top  of  the  cylinder,  —  the  steam  j^w.  79. 


64  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

cock  S  is  opened,  and  the  injection  cock  K  is  closed ;  then  the  steam, 
having  a  pressure  a  little  above  that  of  the  atmosphere,  flows  from  the 
boiler  into  the  cylinder,  and  drives  out  the  air  through  the  snifting  valve 
N.  When  the  cylinder  is  completely  filled  with  steam,  the  steam  cock 
S  is  closed,  and  the  injection  cock  K  is  opened ;  then  a  jet  of  cold  water 
is  thrown  into  the  cylinder,  which  instantly  condenses  the  steam ;  a 
vacuum  being  thus  formed,  the  pressure  of  the  atmosphere  upon  the  top 
of  the  piston  causes  it  to  descend.  "When  the  piston  has  arrived  at  the 
bottom  of  the  cylinder,  the  steam  cock  S  is  again  opened,  and  the 
injection  cock  K  closed;  then  the  steam  again  enters  the  cylinder, 
blows  out,  as  before,  any  air  that  may  have  got  in,  and  forces  the  water 
formed  in  the  cylinder  by  the  condensation  of  the  steam  down  the  educ- 
tion pipe  Q ;  this  water  escapes  by  the  valv.e  D  into  the  cistern ;  the 
steam  beneath  the  piston  now  balances  the  pressure  of  the  external  air, 
and  a  counterpoise  at  the  opposite  end  of  the  great  beam  raises  the  piston 
in  the  cylinder.  But  in  the  engine  represented  by  Fig.  78  this  is  effected 
by  the  momentum  of  the  fly  wheel  Q  Q.  In  the  same  manner  any 
number  of  strokes  are  performed. 

In  this  engine  the  pressure  of  the  atmosphere  is  the  moving  power,  the 
steam  being  merely  employed  to  form  the  vacuum  beneath  the  piston. 
With  the  crank  and  fly  wheel  this  engine  was  employed  as  a  prime 
mover  of  machinery  generally,  and  the  whole  of  its  parts  were  made 
self-acting  by  Beighton  and  Smeaton.  Its  defects  are  as  follows  :  1.  The 
want  of  uniformity  in  the  action  of  the  moving  power  ;  2.  The  loss  of 
power,  at  every  upward  stroke  of  the  piston,  from  the  condensation  of 
steam  by  the  cold  cylinder ;  for  it  will  be  observed  that  at  every  down- 
ward stroke  the  cylinder  had  to  be  cooled  down  by  the  injection  water. 
These  defects  are  completely  remedied  in  Watt's  double-acting  engines, 
by  introducing  a  separate  vessel,  called  the  condenser,  where  the  steam 
is  condensed,  and  by  using  the  steam  not  merely  to  form  a  vacuum,  but 
also  to  move  the  piston  up  and  down  by  its  elastic  pressure. 

Watt's  Engine. 

4.  The  first  engine  constructed  by  Watt  was  what  is  called  the  at- 
mospheric engine,  which  only  differed  in  principle  from  Newcomen's  by 
having  the  steam  condensed  in  a  vessel  separate  from  the  cylinder.  He 
afterwards  employed  the  steam  to  produce  an  upward  as  well  as  a  down- 
ward stroke,  and  from  this  circumstance  the  engine  has  been  called  the 
double-acting  condensing  engine.  This  new  principle  required  that  the 
piston  rod  should  be  connected  with  the  extremity  of  the  great  beam  in 
such  a  manner  that  the  motion  of  the  piston  should  be  communicated  to 
the  beam  in  both  directions  of  the  stroke.  This  led  to  the  invention  of 


U  P 

OF  THE     '       \ 

1    UMVERSiTY  ) 

THE    STEAM    ENGINfc^CU,  ,  65 


the  parallel  motion  described  at  page  55  of  this  work.  Various  other 
mechanical  artifices  were  also  introduced  by  him,  to  render  the  machine 
perfect  in  all  its  parts ;  such  as  the  contrivances  for  lifting  the  valves  so 
as  to  distribute  the  steam  above  and  below  the  piston. 

HIGH   AND    LOW    PRESSURE   ENGINES. 

Steam  engines  are  of  two  kinds  —  the  high  pressure  or  non- condensing 
engine,  and  the  low  pressure  or  condensing  engine.  In  the  high  pressure 
engine,  after  the  steam  has  been  admitted  to  the  cylinder  to  press  on  one 
side  of  the  piston,  forcing  it  up  or  down  according  as  it  enters  from  below 
or  above,  it  escapes  by  a  tube  into  the  open  air.  The  resistance  of  the 
atmosphere  to  the  issue  of  the  steam  diminishes  the  working  force  of  the 
piston.  In  the  low  pressure  engine,  the  escape  pipe,  instead  of  opening 
into  the  air,  is  conducted  into  a  vessel  called  the  condenser,  into  which 
cold  water  is  constantly  running  to  condense  the  steam.  Hence,  as  the 
interior  of  the  low  pressure  engine  is  kept  in  a  state  of  vacuum,  except 
where  the  steam  is  acting,  there  is  no  loss  of  power  by  atmospheric  re- 
sistance ;  and  consequently  a  lower  pressure  of  steam  is  required  to  pro- 
duce an  effect  equal  to  that  of  the  high  pressure  engine. 

As  all  the  condensing  apparatus  is  dispensed  with  in  the  high  pressure 
engine,  it  occupies  less  space,  is  much  less  complicated,  and  is  therefore 
used  on  railroads  for  locomotives. 

VALVES    FOR   REGULATING   THE   DISTRIBUTION   OF   THE 
STEAM    THROUGH    THE    CYLINDER. 

5.  There  are  various  contrivances  now  in  use  for  regulating  the  dis- 
tribution of  the  steam.     In  the  engines  constructed  by  "Watt,  the  valves 
were  opened  and  closed  by  means  of  pins,  or  tappets,  fixed  to  an  oscil- 
lating rod,  called  the  plug  tree,  attached  to  the  great  beam  of  the  engine. 
In  engines  of  moderate  power,  much  more  simple  contrivances  have  been 
adopted,  such  as  the  slide  valve,  the  D  valve,  and  the  four- way  cock. 

Slide    Valves,  fyc. 

6.  Locomotive  Engine,  with  the  common  Slide  Valve.     Fig.  81  repre- 
sents the  common  slide,  with  its  relation  to  the  other  parts  of  the  engine, 
as  commonly  used  in  our  locomotives.     Here  P  is  the  piston,  moving  in 
the  cylinder  C,  which  in  a  locomotive  engine  lies  in  a  horizontal  position ; 
C  D  is  the  piston  rod  passing  through  the  stuffing  box  K  ;  D  E  is  the 
connecting  rod,  being  connected  with  the  piston  rod  by  a  joint  at  D ;  E 
F  is  the  crank  attached  to  the  axle  F  of  the  driving  wheel  of  the  car- 
riage.    The  effect  of  this  mechanical  arrangement  is,  that  whilst  the 

6* 


66 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


piston  moves  backwards  and  forwards  in  the  cylinder,  the  connecting 

rod  and  crank  transmit  this  motion,  so  as  to  give  a  rotatory  motion  to  the 

axle  of  the  driving  wheels,  which  moves 

the  carriage  forward  on  the  rail.     We 

have  now  to  consider  a  distinct  piece  of 

mechanism  for  moving  the  slide  valve  up 

and  down  in  the  steam  box  A  B,  so  as  to 

regulate  the  distribution  of  the  steam  in 

its  passage  into  the  cylinder.    The  motion 

of  the  slide  valve  must  be  so  adjusted  that 

•when  the  piston  is  ascending,  the  steam 

must  be  entering  the  under  part  of  the 

cylinder,  while  the  steam  above  the  piston 

is  allowed  to,  escape  into  the  atmosphere, 

as  in  the  case  of  a  high  pressure  engine, 

or  allowed  to  pass  into  the  condenser  in 

a  condensing  engine ;   on  the  contrary, 

when  the  piston  is  descending,  the  steam 

must  be  entering  the  upper  part  of  the 

cylinder,  while  the  steam  below  the  piston 

is  allowed  to  escape  into  the  atmosphere, 


Fig.  83.  Fig.  84. 


Fig.  82. 


Fig.  81. 


or  the  condenser,  as  the  case  may  be.  In  Fig.  81  A  B  is  the  steam  box, 
which  is  kept  constantly  filled  with  steam  by  the  steam  pipe  S,  proceed- 
ing from  the  boiler  ;  the  slide  valve  is  moved  up  and  down  by  the  rod  R, 
H  passing  through  a  stuffing  box  R  ;  a  is  the  upper  steam  port  or  orifice 
leading  into  the  top  of  the  cylinder,  and  e  is  the  lower  steam  port ;  ex- 
actly between  these  ports  is  an  opening  c,  which  conducts  the  steam  into 
the  condenser,  or  the  atmosphere,  as  the  case  may  be  ;  G  is  an  eccentric 
wheel,  which  turns  upon  the  axle  F  as  a  centre  of  motion ;  G  k  is  the 
eccentric  rod  attached  to  the  extremity  of  the  lever  k  H,  turning  on  the 
fixed  centre  I ;  the  extremity  H  of  this  lever  is  attached  to  the  rod  of 


THE    STEAM    ENGINE.  67 

the  slide  valve,  so  that  when  the  piston  P  is  ascending,  the  slide  valve  is 
descending,  and  vice  versa.  The  slide  valve  is  a  piece  of  metal  hollowed 
on  one  face,  and  made  to  connect  two  of  the  openings,  a  c  e,  on  the  side 
of  the  cylinder,  at  one  time.  Fig.  84  shows  a  separate  longitudinal  sec- 
tion of  the  valve,  and  Fig.  83  shows  a  view  of  its  hollowed  face.  This 
face  lies  flat  against  the  side  of  the  cylinder,  so  that  the  steam,  in  the 
steam  box,  cannot  pass  beneath  the  face  of  the  valve. 

In  Fig.  81  the  piston  P  is  supposed  to  be  ascending,  and  the  steam  is 
passing  through  the  lower  port  e  into  the  under  part  of  the  cylinder,  at 
the  same  time  the  steam  is  passing  from  the  upper  part  of  the  cylinder 
through  the  upper  port  a,  and  is  discharged  through  the  centre  port  c. 
"When  the  piston  has  performed  an  upward  stroke,  and  begins  to  descend, 
as  in  Fig.  82,  the  valve  has  made  a  downward  stroke,  and  now  connects 
the  lower  steam  port  e  with  the  centre  port  c,  leaving  the  upper  port  a 
open  for  the  steam  to  enter  the  upper  part  of  the  cylinder  ;  and  so  on  to 
any  number  of  strokes. 

In  practice  it  is  customary  to  have  the  motion  of  the  valve  so  adjusted 
that  the  steam  port  may  be  slightly  open  when  the  piston  has  completed 
its  stroke.  The  small  space  thus  open  is  called  the  lead  of  the  valve. 
This  lead  allows  time  for  the  steam  to  enter  the  cylinder,  so  as  to  prepare 
for  the  succeeding  stroke  of  the  piston. 

The  D  Valve. 

7.  Figs.  87  and  88  represent  sections  of  this  valve  at  different  po- 
sitions of  the  piston. 

Fig.  85  represents  a  longitudinal  section  of  the  valve  itself.  O  p 
is  the  valve  rod  working  through  the  stuffing  box ;  E  is  an  open- 
ing passing  through  the  valve,  of  which  a  transverse  or  cross  sec- 
tion is  shown  in  Fig.  86  ;  S  is  the  hollow  in  the  valve  through 
which  the  steam  passes  to  the  top  or  bottom  of  the  cylinder,  as 
the  case  may  be  ;  a  b,  in  Fig.  86,  is  the  packing  at  the  back  of 
the  valve,  which  works  steamtight  against  the  valve  box.  This 
is  called  the  D  valve,  from  the  form  of  the  cross  section,  shown 
in  Fig.  86. 

In  the  position  of  the  valve  shown  in  Fig.  88,  the  steam 
passing  through  the  hollow  of  the  valve  enters  the  lower  part 
of  the  cylinder  through  the  port  D,  while  at  the  same  time   gj     |[ 
the  steam  in  the  upper  pgfrt  of  the  cylinder  escapes  through   ^ 
the  port  F,  and,  descending  through  the  longitudinal  opening 
E  in  the  valve,  enters  the  eduction  pipe  c  leading  to  the  con- 
denser.    Fig.  87  represents  an  intermediate   position   of  the 
valve.     The  valve  is  moved  by  an  eccentric,  precisely  in  the 
same  manner  as  described  at  page  56. 


68 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


^T 

Fig.  87. 


r.  88. 


Four-way  Cock. 


8.  Figs.  89   and  90  represent  this  simple  mode  of  distributing  the 
steam.     S,  B,  C,  T  are  four  tubes  ;    S  communicates  with  the  steam 


C        S 


pipe  proceeding  from  the  boiler  ;  C  leads  into  the  condenser,  or  to  the 
external  air,  according  as  the  engine  is  a  condensing  or  a  high  pressure 
one ;  B  leads  to  the  bottom  of  the  cylinder,  and  T  to  the  top  of  it. 
These  four  tubes  enter  a  cock  which  has  two  curved  passages  leading 
through  it,  as  shown  in  the  figures.  These  passages  are  cut  in  such  a 
manner  that  by  turning  the  cock  they  may  be  made  to  open  a  commu- 
nication between  any  two  adjacent  tubes.  In  the  position  of  the  cock 
shown  in  Fig.  90,  the  steam  is  passing  through  the  tube  B  to  the  bottom 


THE    STEAM    ENGINE. 


69 


of  the  cylinder  ;  at  the  same  time  the  steam  is  passing  from  the  top  of 
the  cylinder,  through  the  tube  T,  into  the  tube  C  leading  to  the  con- 


denser. In  Fig.  89  the  cock  has  performed  a  quarter  of  a  revolution  : 
the  steam  is  now  passing  through  the  tube  T  to  the  top  of  the  cylinder  ; 
at  the  same  time  the  steam  is  passing  from  the  bottom  of  the  cylinder 


70      NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 

through  the  tube  B,  into  the  tube  C  leading  to  the  condenser.  The 
eccentric  is  usually  employed  to  move  the  cock  after  the  manner  de- 
scribed. 

GENERAL    VIEW    OF   A    DOUBLE-ACTING    CONDENSING 
ENGINE,  WITH  .THE    FOUR-WAY    COCK. 

9.  The  steam  boiler  and  its  appendages  are  represented  in  Pig.  91. 
Here  F  is  the  furnace ;  B  B  the  water  in  the  boiler  ;  W  W  the  space 
occupied  by  the  steam  ;  Q  is  the  steam  pipe  which  conducts  the  steam  to 
the  cylinder  ;  c  a  b  w  the  safety  valve,  (see  page  58  ;)  O  V  the  pipe  of 
the  water  regulator,  S  being  the  float,  &c.,  (see  page  59  ;)  i  a  pipe,  pro- 
ceeding from  the  hot  water  well,  which  supplies  the  boiler  with  water  ; 
c  and  c'  are  the  water  gauges  ;  the  former,  c,  is  called  the  water  cock,  be- 
cause it  communicates  with  the  wrater  in  the  boiler,  whereas  c'  is  called 
the  steam  cock,  because  it  communicates  with  the  steam  in  the  boiler. 
"When  the  water  in  the  boiler  stands  at  a  proper  level,  upon  opening  the 
two  cocks,  water  will  issue  from  the  water  cock  c,  and  steam  from  the 
steam  cock  c1 ;  but  if  the  boiler  contains  too  little  water,  the  steam  will 
issue  from  both  cocks ;  t  t  is  another  form  of  the  water  gauge,  (see 
page  60.) 

The  engine,  with  its  various  parts,  is  represented  in  Fig.  92.  Here 
B  F  is  the  great  beam  turning  on  the  centre  A  ;  B  K  the  parallel  mo- 
tion, (see  page  55  ;)  E  P  the  piston  rod,  attached  to  the  piston  P  ;  C  the 
cylinder ;  S  the  steam  pipe  transmitting  steam  through  the  four- way 
cock  to  the  top  and  bottom  of  the  cylinder,  as  explained  at  page  68  ;  J 
is  the  condenser,  and  O  the  air  pump,  surrounded  by  the  cold  water  in 
the  cold  water  well  L  L  ;  W  is  the  hot  water  well,  from  which  water  is 
pumped  through  the  pipe  i  i  to  the  reservoir  V,  which  supplies  the  boiler 
with  hot  water  as  it  is  required  ;  N,  the  rod  working  this  pump,  is  at- 
tached to  the  great  beam  ;  M  is  another  rod,  attached  to  the  great  beam, 
working  the  pump  S,  which  supplies  the  cold  water  well  with  a  constant 
stream  of  cold  water ;  F  B,  is  the  connecting  rod  and  crank,  giving  a 
rotatory  motion  to  the  fly  wheel  H  H,  (see  page  53  ;)  the  eccentric,  fixed 
to  the  axle  of  the  crank,  as  shown  in  the  figure,  works  the  four- way 
cock,  as  explained  at  page  56,  &c. ;  G  is  the  governor,  regulating  the 
supply  of  steam  to  the  cy under,  as  explained  at  page  57,  &c. 


HYDROSTATICS  AND   HYDRAULICS. 

1.  HYDROSTATICS  is  that  part  of  Natural  Philosophy  which 
treats  of  the  weight  and  pressure  of  liquids  in  a  state  of  rest ; 
and  HYDRAULICS  treats  of  liquids  in  a  state  of  motion. 

2.  Fluid  bodies  differ  from  solids  in  readily  yielding  to  any  pressure 
applied  to  them,  and  in  their  tendency  to  flow  through  any  channel. 
Solids  tend  towards  the  earth  in  masses  or  lumps  ;  whereas  every  particle 
composing  a  fluid  is  separately  acted  upon  by  the  force  of  gravity.    This 
peculiar  property  of  fluids  depends  upon  the  very  slight  force  of  cohesion 
subsisting  between  their  particles,  which  allows  them  to  have  a  free  motion 
amongst  themselves.     Water  and  air  are  the  most  familiar  examples  of 
fluid  bodies. 

3.  Substances  differ  very  much  in  their  degree  of  fluidity,  or  tendency 
to  flow ;  thus,  water  and  spirits  are  more  fluid  than  oil  or  tar,  and  airs 
or  gases  have  a  higher  degree  of  fluidity  than  water.     Liquids,  such  as 
water,  may  be  poured  from  one  vessel  to  another ;  but  airs  or  gases  are 
so  elastic,  that  they  cannot  be  kept  in  open  vessels.     Thus  the  particles 
of  liquids  are  held  together  by  a  slight  force  of  cohesion ;  whereas  the 
particles  of  airs  repel  each  other,  or  have  a  tendency  to  fly  away  from 
each  other.     The  little'  globules  of  dew  often  seen  on  the  leaves  of  plants 
show  that  the  particles  of  water  have  a  greater  attraction  for  each  other 
than  they  have  for  the  leaf.     A  dry  needle,  gently  placed  upon  the  sur- 
face of  still  water,  will  float,  in  conseqiience  of  its  weight  not  being  suf- 
ficient to  break  the  cohesion  of  the  fluid  particles  on  the  surface. 

4.  Moreover,  whilst  gases  admit  of  being  reduced  in  bulk  by  a  force 
of  compression,  liquids  can  scarcely  be  compressed  at  all. 

Thus,  let  us  suppose  that  P  is  a  piston,  or  plug,  exactly 
fitting  the  smooth  face  of  a  cylinder  A  B  C  D,  and  first 
let  the  space  A  B  C  D  beneath  the  piston  be  filled  with 
common  air ;  then  a  force  of  pressure  applied  to  the  handle 
will  cause  the  piston  to  descend,  and  thereby  to  compress 
the  air  beneath  it ;  but  the  instant  this  pressure  is  with-  , 
drawn,  the  air,  by  its  elasticity,  raises  the  piston,  and  re- 
gains its  original  bulk;  hence,  air  is  called  an  elastic 
fluid.  Again,  let  the  space  A  B  C  D  beneath  the  piston 

be  filled  with  water;  then,  however  great  the  pressure  c 

applied  to  the  handle  maybe,  the  water  beneath  the  piston        Fig.  1. 

(71) 


72      NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 

will  not  be  sensibly  altered  in  its  bulk.  Hence,  water  and  other  liquids 
are  called  non- elastic  *  fluids. 

5.  Three  laws  obtain  in  reference  to  fluid  bodies  :  — 
First.  The  surface  of  still  water  is  always   horizontal  or 

level.  Second.  Fluids  transmit  pressure  equally  in  all  direc- 
tions. Third.  The  pressure  of  water  is  in  proportion  to  its 
depth. 

6.  The  surface  of  still  water  is  always  level. 

Whatever  may  be  the  shape  of  a  mass  of  water,  its  surface,  at  all 
parts,  will  stand  at  the  same  level  or  height.  Thus,  in  Fig.  2,  A  repre- 
sents a  vessel  containing  water,  a  c  a  bent  pipe 
proceeding  from  the  lower  part  of  this  vessel ; 
now,  the  water  in  A  and  the  water  in  a,  having 
a  communication  with  each  other,  stand  at  the 
same  level,  h  z.  In  the  common  teapot,  the 
liquid  in  the  pot  will  always  be  at  the  same 
level  with  that  which  is  in  the  spout.  Work- 
men express  this  property  by  saying  that  "  water  always  finds  its  level." 
On  this  principle,  pipes  are  used  to  convey  water  from  one  place  to 
another,  however  uneven  the  surface  of  the  ground  between  the  places 


Fig.  3. 

may  be.  Towns  and  cities  are  thus  supplied  with  water  from  distant 
springs.  In  Fig.  3,  S  represents  a  spring  or  fountain,  A  B  C  a  pipe 
conveying  water  from  it  to  a  house ;  now,  although  this  pipe  may  rise 


*  This  is  not  strictly  true,  because,  under  a  pressure  of  15  Ibs.  per  square 
inch,  water  is  reduced  about  the  22,000th*  part  of  its  bulk ;  however,  for  all 
ordinary  pressures,  water  may  be  practically  regarded  as  non-elastic. 


HYDROSTATICS    AND    HYDRAULICS. 


73 


over  hills,  or  pass  through  valleys,  yet  so  long  as  it  does  not  rise  above  the 
level  of  the  fountain  head,  the  water  will  continue  to  flow.  On  this 
principle  the  city  of  Philadelphia  is  supplied  with  water  from  Fairmount 
reservoir,  New  York  from  the  Croton  River,  and  Boston  from  the  Co- 
chituate  Lake.  On  this  principle,  also,  little  streams  descend  from  the 
hills  to  the  plains,  and  mighty  rivers  flow  on  towards  the  ocean. 

Levelling. 

7.  The  heights  of  mountains,  as  given  in  geography,  are  always 
estimated  from  the  level  of  the  ocean.  If  the  earth  were  an  exact 
sphere,  which  it  is  very  nearly,  the  surface  of  the  ocean  would  be  every 
where  at  the  same  distance  from  the  earth's  centre.  The  surface  of  a 
large  extent  of  water  is  consequently  convex ;  but  for  any  small  extent 
of  water,  the  surface  is  practically  flat.  A  spirit  level  is  the  instrument 
which  is  usually  employed  for  finding  the  difference  of  levels  between 
two  places.  It  consists  of  a  glass  tube  E  F,  filled  v  B  % 

with  spirits  of  wine,  excepting  a  small  part  B,     fig 
called  the  air  bubble.     This  bubble  always  rises  to  j^y.  4. 

the  higher  end  of  the  tube  ;  but  when  the  tube  is 
perfectly  level,  the  bubble  stands  at  the  middle,  B.     In  using  this  instru- 


Fig.  5. 

ment  for  ordinary  purposes,  such  as  finding  the  levels  for  buildings,  or 
drains,  it  is  fixed  in  a  frame  with  sights  placed  over  the  direction  of  the 
tube ;  but  when  levels  are  to  be  taken  for  great  distances,  such  as  occur 
7 


74 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


in  the  construction  of  railways,  the  instrument  is  fixed  upon  the  top  of  a 

spy  glass,  or  telescope,  mounted  on  a  tripod  stand,  as  in  the  accompany- 

ing figure,  where  E  F  is  the  spirit  level  ;  A  B  the  spy  glass  fixed  in  the 

horizontal  plate  C  D  ;  screws  are  placed  below  this  plate  for  adjusting 

the  spirit  level  ;  L  L  L  are  portions  of  the  legs  on  which  the  instrument 

stands  ;  the  spy  glass,  with  the  spirit  level,  turns  round  on  a  vertical 

axis,  so  that  the  person  using  the  instrument  can  direct  the  spy  glass 

towards  any  object.     It  will  be  observed  that  the  line  of  vision  through 

the  spy  glass  is  a  level  line,  for  it  is  exactly  parallel  to  the  level  line 

formed  by  the  spirit  level.     In  order  to  show  the  manner  of  using  this 

instrument,  let  it  be  required  to  ascertain  whether  there  is  a  proper  de- 

scent for  water  from  a  well  at 

F  to  a  village  at  B  :  place  the 

spirit  level  D  on  the  eminence 

E,  from  which  levelling  staves 

F  and  G  B  can  be  seen  ;  ad- 

just the  level,  and  direct  the 

spy  glass  to  the  staff"  F  ;  turn 

the  spy  glass  upon  its  vertical 

axis,  and  direct  it  to  the  staff 

G  B  ;   then  the  difference  be- 

tween the  heights  of  the  staves  G  B  and  F  will  give  the  descent  of  the 

water  from  the  well  to  the  place  B. 

8.  The  line  which  is  determined  by  the  spirit  level  is  a  tangent  to  the 
earth's  surface  ;  in  taking  levels,  therefore,  in  this  manner  for  any  great 
distance,  an  allowance  must  be  made  for  the  convexity  of  the  earth  ; 
this  allowance  is  about  8  inches  for  a  mile.  In 
Fig.  7,  D  A  C  represents  a  portion  of  the  earth's 
surface,  or  the  form  which  the  water  of  the  ocean 
assumes  ;  A  B  is  the  apparent  level  taken  from 
A,  or  the  line  determined  by  the  spirit  level  ; 
A  C  is  the  true  level,  or  the  surface  which  water  extending  from  A  to  C 
would  assume  ;  B  C  is  the  correction,  or  the  difference  between  the  ap- 
parent level  and  the  true  level,  which  is  about  8  inches,  when  the  dis- 
tance A  C  is  one  mile. 

9.    Fluids  transmit  pressure  equally  in  all  directions. 

In  order  to  exemplify  this  principle,  let  us  suppose  a  bladder  to  be 
filled  with  water,  and  after  the  mouth  is  closed,  let  it  be  squeezed  or 
pressed  with  a  force  nearly  sufficient  to  burst  it  ;  now,  every  particle  of 
the  water  will  undergo  the  same  amount  of  pressure,  and  every  part  of 
the  bladder  will  be  pressed  upon  by  the  water  with  the  same  force. 


7. 


HYDROSTATICS    AND    HYDRAULICS. 


75 


Fig.  8. 


10.  In  the  syringe,  Fig.  8,  the  piston  or  plug  P  is  forced 
down  upon  the  water  beneath  it,  and  the  pressure  thus  pro- 
duced is  transmitted  equally  through  the  whole  body  of  the 
fluid ;  hence  it  is  that  the  water  is  driven  out  at  the  orifice 
O  with  a  force  corresponding  to  the  pressure  applied  to  the 
piston. 

11.  The  principle  of  the  hydrostatic  press  depends  upon 
this  property  of  fluids  :  In  Fig.  9,  A  and  a  are  two  cylinders 
containing  water,  connected  by  a  pipe ;  P  is  a  piston  fitting 
the  large  cylinder,  and  p  another  piston  fitting  the  small  one. 
Now,  any  pressure  applied  to  the  small  piston  will  be  trans- 
mitted by  the  water  to  the  large  piston,  so  that  every  portion 
of  surface  in  the  large  piston  P  will  be  pressed  upwards  with 
the  same  force  that  an  equal  portion  of  surface  in  the  small 
piston  p  is  pressed  downwards.     For  example,  let  p  contain 

one  inch  of  surface,  and  let  the  downward  pressure  applied  to  it  be  20 
Ibs. ;  then  every  inch  of  surface  in  P  will  be 
pressed  upwards  with  the  force  of  20  Ibs.,  and 
therefore  as  many  times  as  the  surface  of  the 
large  piston  is  greater  than  that  of  the  small  one, 
just,  so  many  times  will  the  upward  pressure 
upon  the  large  piston  be  greater  than  the  down- 
ward pressure  upon  the  small  one;  thus  if  P 
contain  a  surface  of  30  inches,  then  the  pressure 
upon  it  will  be  equal  to  30  times  20  Ibs.  or  600  Ibs. 

12.  The  pressure  of  water  is  in  proportion  to  its  depth. 

As  all  the  particles  of  a  fluid  press  on  those  immediately  below  them, 
the  particles  at  any  given  depth  will  have  to  sustain  the  weight  or 
pressure  of  those 'which  lie  above  them.    Thus,  Fig.  10,  the  particles  at 
the  bottom  I  J  of  a  vessel  filled  with  water  will  sus- 
tain double  the  pressure  of  those  lying  in  the  middle 
E  F ;  and  the  particles  at  this  depth  will  sustain  double 
the  pressure  of  those  lying  in  C  D  at  one  fourth  the 
whole  depth  ;  and  so  on  ;  hence  the  pressure  of  water 
at  any  depth  is  in  proportion  to  that  depth.     But  as 
water  transmits  pressure  equally  in  all  directions,  this  GJ 
pressure  will   act   sideways   as   well   as   downwards. 
Hence  the  pressure  at  the  points  A,  C,  E,  G,  and  K 
will  be  as  the  numbers  0,  1,  2,  3,  and  4  — that  is,  as 
the  depths.     Let  holes  be  bored  of  the  same  size  at 


10' 


the  points  D,  F,  H,  J,  and  K  ;  then  the  water  will  flow  out  at  these 


76  NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

holes  with  forces  proportioned  to  the  pressure  of  the  water  at  these 
points  —  that  is,  with  forces  in  proportion  to  the  depth  of  the  holes  be- 
low the  surface  A  B  of  the  water :  thus  the  water  will  flow  from  K  and 
J  with  the  same  force ;  from  F  it  will  flow  with  half  the  force  that  it 
does  from  K  or  J  ;  from  D  it  will  flow  with  one  fourth  the  force  that  it 
does  from  J ;  and  so  on. 

Pressure  on  the  Bottom  of  Vessels. 

13.  In  upright  vessels,  the  pressure  on  the  bottom  is  ob-  A 
viously  equal  to  the  weight  of  the  water:  thus,  in  Fig.  11, 
the  pressure  on  the  bottom  D  C  is  equal  to  the  weight  of  the 
water  F  G  C  D  contained  in  the  vessel ;  hence  the  pressure 
upon  the  bottom  of  a  vessel  is  found  by  multiplying  together 
the  area  of  the  base,  the  perpendicular  depth  of  the  water,  and 
the  weight  of  a  cubic  foot  of  water. 

An  example  will  render  the  truth  of  this  rule  more  apparent. 

Example.  Let  the  area  of  the  base  D  C  contain  two  square  feet,  and 
let  the  depth  of  the  water  G  C  be  three  feet ;  required  the  pressure  on 
the  bottom  of  the  vessel,  the  weight  of  a  cubic  foot  of  water  being 
1000  oz. 

Content  of  water  =  area  base  X  perpendicular  height. 
=  2X3  =  6  cubic  feet. 

Weight  of  water,  or  pressure  on  base,  =  6  X  1000  =  6000  oz. 

14.  The  pressure  on  the  bottom  of  a  vessel,  whatever  may 
be  its  form,  depends  solely  upon  the  area  of  the  base  and  the 
perpendicular  depth  of  the  water.     This  arises  from  the  law 
of  equal  distribution  of  pressure  explained  in  Art.  9. 

Figs.  12,  13,  and  14,  represent 
three  different  vessels  having  equal 
bases  and  the  same  perpendicular 
depth  of  water  in  them  ;  all  their 
bases  will  sustain  the  same  amount  of  j?ig,  12.  13.  14. 

pressure.   Hence  the  pressure  on  the 

bottom  of  a  vessel  containing  water  is  equal  to  the  weight  of  a  column 
of  water  whose  base  is  equal  to  the  bottom  of  the  vessel,  and  its  height 
equal  to  the  depth  of  the  bottom  from  the  surface  of  the  water :  thus  in 
the  three  forms  of  vessels  represented  in  the  foregoing  cut  the  pressure 
on  the  bottom  is,  in  all  the  cases,  equal  to  the  weight  of  the  column  of 
water  A  B  C  D. 


HYDROSTATICS    AND    HYDRAULICS. 


77 


Upward  Pressure  of  Water. 

15.  The  upward  pressure  of  water  is  very  clearly  proved  by  the  fol- 
lowing simple  experiments  :  — 

EXPERIMENTS. 

Exp.  1.  Tie  a  piece  of  bladder  B  (Tig.  15)  to  one  end  of  an 
open  tube  A  B  ;  pour  water  into  it ;  then,  owing  to  the  pres- 
sure of  this  water,  the  bladder  becomes  convex  ;  dip  the  tube 
into  a  vessel  of  water;  as  the  tube  is  depressed  the  bladder  be- 
comes less  and  less  convex,  and  it  becomes  perfectly  flat  when 
the  water  in  the  tube  is  on  a  level  with  the  water  in  the  ves- 
sel, for  then  the  upward  pressure  of  the  latter  is  equal  to  the 
downward  pressure  of  the  former ;  when  the  tube  is  plunged  Fig,  15. 
deeper  than  this,  the  bladder  becomes  concave. 

Exp.  2.  In  Fig.  16,  A  B  is  a  thick  tube  having  its 
under  end  ground  straight ;  n  is  a  flat  piece  of  lead 
having  a  string  n  H  attached  to  it,  so  that  the  plate  of 
lead  may  be  drawn  close  to  the  end  of  the  tube ;  plunge 
the  tube  into  a  vessel  of  water ;  quit  the  string :  the 
plate  remains  supported  by  the  upward  pressure  of  the 
water;  raise  the  tube  until  the  lead  falls  off;  at  this 
point  the  depth  of  the  lead  below  the  surface  of  the 
fluid  will  be  about  eleven  times  its  thickness,  for  lead 
is  about  eleven  times  the  weight  of  an  equal  bulk  of 
water. 

Exp.  3.   Dip  the  end  B  of  a  small  tube  (Fig.  17)  into 
quicksilver ;  stop  the  upper  end  A  with  the  finger  and 
lift  up  the  tube  from  the  quicksilver :  a  small  column  of 
quicksilver  remains  in  the  lower  end  of  the  tube.     Plunge 
the  end  of  the  tube  into  a  vessel  of  water  to  a  depth  rather 
more  than    13^  times  the  length   of  the  little  column  of 
quicksilver,  then  take  away  the  finger :  the  quicksilver  re- 
mains supported  by  the  upward  pressure  of  the  water.     It 
will  be  observed  that  quicksilver  is  about  13£  times  the 
weight  of  an  equal  bulk  of  water. 

16.  The  upward  pressure  of  a  long  column  of  water  is 
strikingly  exhibited  in  the  hydrostatic  bellows.    Two  boards, 

A  and  B,  Fig.  18,  are  connected  together  by  means  of       Fig.  17. 
leather,  as  in  the  common  bellows ;  a  small  pipe  a  b  c  com- 
municates with  the  inside  of  the  bellows  ;  a  heavy  weight  W  is  placed 
upon  the  upper  board  to  show  the  effect  of  the  pressure.      "Water  is 
poured  in  at  the  mouth  c,  so  as  to  fill  the  bellows  as  well  as  the  tube. 
7* 


Fig.  16. 


78 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  18. 


Now,  when  water  is  poured  into  the  tube,  the  upper 
board  A,  with  its  weight  W,  is  lifted  up  by  the  pres- 
sure of  the  water  beneath  it.  The  bore  of  the  pipe 
may  be  as  small  as  you  please,  for  the  power  of  the 
instrument  merely  depends  upon  the  height  of  the 
column  of  water  b  c  in  the  small  tube  and  the  area  of 
the  board  A  B ;  that  is  to  say,  the  weight  W  which  is 
raised  is  equal  to  the  weight  of  a  column  of  water 
A  b  c  standing  on  the  upper  board  as  a  base  and  hav- 
ing b  c  as  its  perpendicular  height.  If  the  area  of  the 
board  A  b  is  one  square  foot,  and  the  height  of  the 
column  of  water  b  c  in  the  small  tube  is  three  feet, 
then  the  upward  pressure  upon  the  board  will  be  equal 
to  the  weight  of  three  cubic  feet  of  water,  or  equal 
to  3000  oz.  Now,  by  making  the  bore  of  the  pipe 
very  small  we  may  suppose  this  effect  to  be  produced  by  one  ounce  of 
water  poured  into  the  pipe  ;  this  ounce  of  water,  therefore,  is  sufficient 
to  counterbalance  three  thousand  ounces  placed  upon  the  bellows  board. 
The  astonishing  effect  of  a  small  column  of  water  acting  in  this  manner 
has  been  called  the  hydrostatic  paradox. 

17.  In  this  manner  a  strong  cask  (Fig.  19)  filled  with  liquid 
may  be  burst  by  a  few  ounces  of  water  poured  into  a  long  tube 
A  communicating  with  the  inside  of  the  cask. 

If  a  strong,  square  glass  bottle,  empty  and  firmly  corked, 
be  sunk  in  water,  its  sides  will  be  crushed  inwards  by  the  pres- 
sure before  it  reaches  a  depth  of  ten  fathoms. 

If  a  corked  empty  bottle  be  let  down  into  the  sea,  the  cork  is 
usually  forced  inwards  at  a  certain  depth. 

When  a  ship  founders  at  sea,  the  great  pressure  at  the  bottom  pig.  19. 
forces  water  into  the  pores  of  the  wood,  and  makes  it  so  heavy 
that  no  part  can  ever  rise  again. 

18.  This  law  of  pressure  is  also  sometimes  seen  acting  on  a  great  scale 
in  nature  in  the  rending  of  rocks  and  mountains.     Let  A  B  (Fig  20) 
represent  a  long  vertical  fissure  or  crevice, 

communicating  with  an  internal  cavity  C 
formed  in  the  mountain,  but  without  any 
outlet ;  now,  when  the  fissure  and  cavity  be- 
come filled  with  water,  an  enormous  upheav- 
ing force  is  produced,  sufficient,  it  may  be, 
to  cause  a  disruption  of  the  mass  D. 

Acting  in  this  way,  water  seems  to  be  one 
of  those  great  agents  in  nature  which  are  con- 
stantly producing  changes  on  the  surface  of 


Fig.  20. 


HYDROSTATICS    AND    HYDRAULICS. 


79 


the  globe.     The  freezing  of  water,  under  the  same  circumstances,  also 
tends  to  produce  similar  effects. 


Fig.  21. 


Pressure  on  the  Sides  of   Vessels. 

19.  It  has  been  shown  in  Art.  12  that  the  pressure  of  water  on  a  point 
in  the  side  of  a  vessel  increases  with  the  depth  of  that  point  below  the 
surface.     Let  A  I  (Fig.  21)  be  the  section  of  the 

side  of  a  rectangular  vessel  filled  with  water,  and 
let  the  whole  depth  A I  be  8  feet ;  then  at  the  mid- 
dle point  E  the  depth  A  E  will  be  4  feet.  Now, 
the  pressure  at  I  is  produced  by  a  column  of  water 
whose  height  is  8  feet,  whereas  the  pressure  at  the 
middle  point  is  produced  by  a  column  whose  depth 
is  4  feet,  which  is  just  the  mean  or  average  between 
the  top  and  bottom  pressures,  and  in  fact  the  aver- 
age of  all  the  pressures  acting  upon  the  side ;  hence  the  whole  pressure 
upon  the  side  will  be  produced  by  a  column  of  water  whose  base  is  the 
area  of  that  side  with  an  average  depth  equal  to  half  the  whole  height 
or  depth  of  that  side.  We  therefore  have  the  following  rule  for  finding 
the  pressure  on  the  side  of  a  vessel :  — 

Multiply  the  area  of  the  side,  in  feet,  by  one  half  its  depth, 
and  this  product  again  by  the  weight  of  a  cubic  foot  of  water. 

Conceive  a  surface  equal  to  the  side  of  the  vessel  to  be  laid  in  the  bot- 
tom, then  the  pressure  on  this  surface  will  be  double  the  actual  pressure 
on  the  side  ;  for  in  this  case  the  column  of  pressure  is  the  whole  depth, 
whereas  the  column  of  pressure  on  the  side  is  only  equivalent  to  half  the 
whole  depth. 

It  is  important  to  observe  that  the  pressure  on  the  side  of  a  vessel  has 
nothing  to  do  with  the  length  of  the  vessel  in  the  direction  A  E,  for  the 
pressure  is  simply  equal  to  the  product  of  the  area  of  the  side,  the  depth 
of  its  middle  point,  and  the  weight  of  a  cubic  foot  of  water  :  thus,  — 

Pressure  on  the  side  of  a  vessel  =  area  side  X  half  depth  X  wt.  c.  ft. 
water. 

20.  In  consequence  of  the  increase  of  pres- 
sure with  the  depth,  embankments  and  dams 
are  made  broader  at  the  bottom  than  at  the 
top,  (Fig.  22.)     And,  on  the  same  principle, 
in  order  to  have  a  cask  equally  strong,  there 
should  be  more  hoops  placed  towards  the  bot- 
tom than  towards  the  top. 


Fig.  22. 


80  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Centre  of  Pressure. 

21.  The  centre  of  pressure  is  that  particular  point  in  the 
side  of  a  vessel  where  the  whole  pressure  upon  it  may  be 
conceived  to  be  applied  without  altering  the  effect. 

Thus  (Fig.  23)  let  the  surface  A  Q  D  G  be  subject  to  the  pressure  of 
•water ;  then  there  must  be  a  point  C  in  that  surface  where  a  single  op- 
posing pressure  may  be  applied  which  shall  exactly    , 
balance  the  whole  pressure  of  the  water  ;  this  point  C 
is  called  the  centre  of  pressure.    This  point  must  obvi- 
ously lie  in  the  vertical  line  E  F,  dividing  the  surface 
equally  ;  moreover  it  must  be  nearer  to  the  bottom 
than  it  is  to  the  top ;  in  fact  its  distance  from  the  bot- 
tom is  found  to  be  one  third  the  whole  depth  ;  that  is 
to  say,  F  C  is  equal  to  one  third  E  F.     An  example          jy^  23. 
will  render  the  subject  more  plain  :  — 

Ex.  Let  the  breadth  A  Q  =  4  feet,  the  depth  A  G  or  E  F  =  9  feet : 
required  the  position  of  the  centre  of  pressure,  and  also  the  whole  pres- 
sure of  the  water. 

Here  the  distance  of  the  centre  of  pressure  from  the  bottom  of  the 
vessel  is  equal  to  J  of  9  feet,  or  3  feet. 

To  find  the  pressure,  we  have 

Area  surface  A  Q  D  G  =  4  X  9  =  36  sq.  feet; 
Pressure  on  A  Q  D  G  =  area  X  h  depth  X  wt.  c.  ft.  water. 
=  36  X  h  of  9  X  1000.  ' 
=  162,000  oz.,  or  10,125  Ibs. 

Here  a  pressure  of  10,125  Ibs.  applied  at  C  would  counterbalance  the 
pressure  of  the  water  upon  the  whole  surface. 

It  may  be  worthy  of  observation,  that  a  single  hoop,  placed  upon  a 
barrel  at  one  third  the  whole  height  from  the  bottom,  would  counterbal- 
ance the  pressure  of  the  liquid  upon  the  staves. 

SPECIFIC  GRAVITY. 

22.  Bodies  differ  very  much  in  their  density,  or  in  the  quantity  of 
matter  which  they  contain  in  a  given  bulk ;  thus  the  weight  of  a  lump 
of  lead  is  more  than  forty  times  the  weight  of  an  equal  bulk  of  cork, 
and  the  weight  of  a  piece  of  platinum  is  nearly  double  the  weight  of  an 
equal  bulk  of  lead.     The  specific  gravity  of  a  body  is  its  weight  as  com- 
pared with  the  weight  of  an  equal  bulk  of  some  other  body,  taken  as  a 
standard.     For  the  sake  of  convenience,  pure  water,  at  the  temperature 
of  60°,  is  taken  as  the  standard  by  which  the  specific  gravities  of  all 
other  substances  are  compared.     Taking,  the  specific  gravity  of  water  as 


HYDROSTATICS    AND    HYDRAULICS. 


81 


unity,  the  specific  gravity  of  any  other  substance  is  expressed  by  the 
number  of  times  that  it  is  heavier  than  an  equal  bulk  of  water :  thus 
iron  is  8  times  the  weight  of  an  equal  bulk  of  water  ;  therefore  the  spe- 
cific gravity  of  iron  is  8  ;  and  so  on  to  other  cases.  Now,  the  weight  of 
a  cubic  foot  of  water  is  exactly  1000  ounces ;  hence  we  find  the  weight  of 
a  cubic  foot  of  any  substance  by  simply  taking  its  specific  gravity  as  so 
many  thousands  of  ounces ;  thus  the  weight  of  a  cubic  foot  of  iron  is 
8000  ounces. 

23.  A  body  sinks  or  floats,  according  as  its  specific  gravity  is  greater 
or  less  than  the  fluid  in  which  it  is  immersed ;  and  when  the  specific 
gravity  of  the  body  is  equal  to  that  of  the  water,  the  body,  upon  being 
immersed,  neither  rises  nor  falls,  but  remains  as  it  were  suspended  in  the 
fluid  at  ail  depths. 

24.  The  most  important  laws,  regulating  the  pressure  of  fluids  on 
solids  immersed  in  them,  are  as  follows :  — 

1.  When  a  solid  body  floats  on  a  fluid,  the  weight  of  the 
fluid  displaced  is  equal  to  the  weight  of  the  body. 

2.  When  a  heavy  body  is  weighed  in  water,  the  weight 
which  the  body  loses  is  due  to  the  upward  pressure  or  buoy- 
ancy of  the  water,  and  is  equal  to  the  weight  of  the  water 
displaced. 

25.  The  following  experiments  are  intended  to  illustrate  these  impor- 
tant laws,  as  well  as  other  properties  of  fluids  depending  upon  their  spe- 
cific gravity. 

EXPERIMENTS. 

Exp.  1.  Fluids  may  be  placed  upon  each  other  in 
the  order  of  then:  specific  gravities;  thus  mercury, 
water,  oil,  and  spirits  may  be  placed  upon  each  other 
in  a  test  tube,  as  in  the  annexed  cut.  (Fig.  24.) 

Exp.  2.  Fluids  may  be  made  to  balance  each  other 
by  their  opposing  pressures,  and  in  such  cases  the 
columns  of  the  fluids  are  reversely  as  their  specific 
gravities.  Take  a  bent  tube,  (Fig.  25,)  introduce  a  little 
mercury  into  one  leg,  and  some  water  into  the  other : 
the  column  of  water  will  be  about  13£  times  the  height 
of  the  mercury,  in  order  that  they  should  balance  each 
other.  From  this  it  follows  that  mercury  is  about  13£ 
times  the  weight  of  water. 

Exp.  3.  Try  water  and  muriatic  ether,  as  in  the  last 
experiment :  the  column  of  ether  will  be  about  !£ 
times  the  column  of  water.  jy«  25. 


82 


NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 


Fig.  26. 


Exp.  4.  A  light  fluid  will  rise  within  a  heavy  one.  Take  a  small 
bottle  filled  with  red  wine,  (or  any  colored  liquor  specifically  lighter 
than  water ;)  invert  it  with  its  mouth  at  the  bottom  of  the  vessel  of 
water :  the  wine  rises  through  the  water. 

Exp.  5.  A  heavy  fluid  will  sink  in  a  lighter  one.  Take  a  small 
bottle  filled  with  diluted  sulphuric  acid,  colored  red  by  the  tincture  of 
litmus ;  invert  the  mouth  of  the  bottle  at  the  top  of  some  hot  water : 
the  heavy  colored  liquid  descends  through  the  water. 

Exp.  6.  Fill  a  vessel  A  (Fig.  26,)  having  an  opening  a,  with  water,  until 
it  begins  to  run  out  at  a ;  place  any  floating  body  W 
on  the  surface  of  the  water ;  the  body  sinks  to  a  cer- 
tain depth,  and  thereby  displaces  a  portion  of  water 
equal  to  the  bulk  of  that  part  of  the  body  which 
is  immersed ;  receive  this  water  in  the  vessel  B ; 
weigh  the  water  thus  received,  and  it  will  be  found 
to  be  equal  to  the  weight  of  the  floating  body. 

Exp.  7.  Again:  fill  the  vessel  A  with  water 
until  it  begins  to  run  out  at  a ;  immerse  any  body 
W  completely  in  the  water,  then  a  quantity  of 
water  will  run  out,  into  the  vessel  B,  equal  in  bulk  to  the  body 
immersed  ;  weigh  this  water,  and  it  will  give  the  weight  of  water  equal 
in  bulk  to  the  body.  The  actual  weight  of  the  body  divided  by  the 
weight  of  this  water  which  it  displaces  will  give  the  specific  gravity  of 
the  body :  thus  if  the  weight  of  the  body  is  6  ounces,  and  the  weight  of 
the  water  which  it  displaces  3  ounces,  then  the  weight  of  the  body  will 
be  2  times  the  weight  of  an  equal  bulk  of  water  ;  that  is,  the  specific 
gravity  of  the  body  will  be  2,  that  of  water  being  1.  This  is  a  simple, 
though  somewhat  rude,  method  of  finding  the  specific  gravity  of  a  body. 

Exp.  8.  Two  equal  weights  A  and  B  (Fig.  27) 
are  duly  balanced  over  a  pulley ;  place  one  of  them 
A  at  the  bottom  of  ^m  empty  vessel ;  pour  water 
into  the  vessel ;  the  equilibrium  is  destroyed  by  the 
buoyancy  or  upward  pressure  of  the  water  upon  the 
weight  A,  and  it  consequently  ascends  to  the  sur- 
face of  the  fluid.  Hence  it  "requires  less  force  to 
raise  a  body  in  water  than  it  does  to  raise  the  same 
body  in  air. 

Exp.  9.   Take  a  small  long-necked  bottle,  and  put 
some  shot  into  it,  so  as  to  make  it  float  to  a  con- 
venient depth  in  water ;  make  a  mark  on  the  neck  of  the 
bottle  at  the  level  of  the  water ;  now  float  the  bottle  in  some 
other  liquid,  such  as  oil  or  beer,  whose  specific  gravity  is  less 

than  that  of  water ;  the  bottle  sinks  to  a  greater  depth. 

Fig.  28. 


HYDROSTATICS    AND    HYDRAULICS. 


83 


Here  the  bulk  of  the  displaced  liquid  is  greater  according  as  its  specific 
gravity  is  less. 

Exp.    10.    To   show  •  that   the   weight   which   a   body   loses   in   water 
is  equal  to   the   weight  of  the  fluid  displaced :  place  a  hollow    cylin- 
der a  011  one   scale  of  a  balance,  and 
suspend  to  this  scale  a  solid  metal  cyl- 
inder b,  which  exactly  fits  the  hollow  t 
formed  in  the  other  cylinder  a ;  place  a 
weight  c  on  the  opposite  scale  so  as  to 
restore  the  balance ;  plunge  b  into  a  ves- 
sel of  water  ;  the  scale  c  descends .:  now 
fill  the  hollow  cylinder  a  with  water ; 
the  equilibrium  is  restored.     It  will  be 
observed  that  the  water  which  restored 
the  equilibrium  is  equal  to  the  bulk  of 
the  body  b. 

Exp.  11.  Balance  a  glass  of  water  in 
a  pair  of  common  scales  ;  suspend  a  two 
ounce  brass  weight  by  a  fine  thread  hold  the  weight,  thus  suspended, 
in  the  water ;  then  the  scale  on  which  the  glass  of  water  is  placed  de- 
scends, and  it  will  require  about  a  quarter  of  an  ounce  to  restore  the 
equilibrium.  This  quarter  of  an  ounce  is  the  weight  of  water  equal  in 
bulk  to  the  brass  weight ;  therefore  brass  is  about  eight  times  the  weight 
of  an  equal  bulk  of  water. 


Fig.  29. 


To  find  the  Specific  Gravity  of  Liquids  by  Means  of  a  small 
Bottle. 

26.  Take  a  small  glass  bottle  having  a  long  neck ;  find  the  weight 
of  this  bottle ;  fill  it  with  pure  water  up  to  a  certain  mark  made  upon 
the  neck ;  weigh  the  bottle  and  water ;  then  the  difference  between 
these  weights  will  give  the  weight  of  the  enclosed  water.  Pour  out  the 
water  and  introduce  the  liquid,  whose  specific  gravity  is  to  be  deter- 
mined, to  the  height  of  the  mark  made  upon  the  neck  of  the  bottle ; 
weigh  the  bottle  and  liquid ;  then  the  difference  between  this  weight 
and  that  of  the  bottle  itself  will  give  the  weight  of  the  enclosed  liquid. 
Now,  having  thus  obtained  the  weight  of  equal  bulks  of  water  and  the 
liquid,  the  latter  weight  divided  by  the  former  will  give  the  specific 
gravity  of  the  liquid.  For  example,  let^the  weight  of  the  empty  bottle 
be  200  grains,  that  of  the  bottle  filled  with  water  800  grains,  and  that 
of  the  bottle  and  liquid  1100  grains ;  then  we  have 


84  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Weight  of  the  water  =  800  —  200  =  600, 
Weight  of  the  liquid  =  1100  —  200  =  900  ; 

Specific  gravity  of  the  liquid  = ji 

600 

To  find  the  Specific   Gravity  of  Bodies  by  the  Hydrostatic 
Balance. 

27.  The  hydrostatic  balance  differs  from  an  ordinary  one   only  in 
having  a  hook  attached  to  the  under  side  of  one 

of  the  scales.  The  body,  whose  specific  gravity  is 
to  be  found,  is  suspended  from  the  hook  by  a  horse 
hair,  and  then  its  weight  is  determined.  It  is 
now  weighed  in  water,  and  thus  its  loss  of  weight 
is  ascertained.  Now,  it  has  been  explained  (see 
Exp.  10)  that  the  weight  which  a  body  loses  in 
water  is  equal  to  the  weight  of  a  portion  of  water 
equal  in  bulk  to  the  body,  and  hence  we  have  the 
following  rule :  — 

The  specific  gravity  of  a  body  is  equal  to  its  weight  di- 
vided by  the  weight  which  it  loses  in  water. 

Ex.  The  weight  of  a  solid  body  is  200  grains,  but  its  weight  in  water 
is  only  150  grains :  required  the  specific  gravity  of  the  body. 

Here  the  loss  of  weight  in  water,  or,  what  is  the  same  thing,  the 
weight  of  water  equal  in  bulk  to  the  body  =  200  —  150  =  50  ;  but 
the  weight  of  the  body  itself  =  200 ; 

The  specific  gravity  of  the  body,  or  the  number  of  tunes  which  it  is 

200 
heavier  than  an  equal  bulk  of  water,  =  —  =  4. 

28.  The  specific  gravity  of  liquids  may  be  found  by  the  hydrostatic 
balance,  in  the  following  manner  :  — 

Weigh  a  solid  body  in  water,  as  well  as  in  the  liquid  whose  specific 
gravity  is  to  be  determined ;  then  the  loss  in  each  case  will  be  the  re- 
spective weights  of  equal  bulks  of  water  and  the  liquid  ;  therefore  — 

The  loss  of  weight  in  the  liquid,  divided  by  the  loss  of 
weight  in  the  water,  will  give  the  specific  gravity  of  the  liquid. 

The  solid  body  used  in  this  process  is  usually  a  heavy  piece  of  glass, 
suspended  from  the  scale  by  means  of  a  fine  platinum  wire. 

Ex.  A  heavy  piece  of  glass  loses  2  ounces  when  weighed  in  water, 
and  3  ounces  when  weighed  in  diluted  sulphuric  acid  :  required  the  spe- 
cific gravity  of  the  acid. 


HYDROSTATICS    AND    HYDRAULICS.  85 

Here  the  weights  of  equal  bulks  of  the  two  liquids  are  2  and  3  ounces 
respectively. 

wt.  acid         „ 
Specific  gravity  of  the  acid  =  wt>  water  =  f  ==  H- 

29.  A  false  gold  coin  may  be  detected  by  finding  its  specific  gravity, 
for  as  pure  gold  has  a  greater  specific  gravity  than  any  of  the  metals, 
such  as  silver  or  copper,  with  which  it  may  be  adulterated,  the  counter- 
feit coin  will  have  a  less  specific  gravity  than  standard  gold.     Trades- 
men use  a  very  simple  method  for  detecting  a  false  coin.     A  standard 
coin  must  have  a  proper  weight,  and  also  a  certain  bulk  corresponding 
to  its  weight ;  now  a  false  coin,  having  the  proper  weight,  will  have  a 
greater  bulk  than  a  true  one ;  hence  the  tradesman  employs  two  tests 
for  ascertaining  a  good  com  ;  he  first  weighs  it,  and  if  this  is  found  cor- 
rect, he  then  tries  to  pass  it  through  a  slit  made  exactly  to  fit  the  thick- 
ness and  diameter  of  a  standard  coin ;  if  the  coin  under  examination 
does  not  pass  through  this  slit,  he  concludes  that  the  coin  is  counterfeit. 

Specific  Gravity  of  Bodies  determined  ly  the  Hydrometer. 

30.  These  instruments  depend  upon  the  principle,  that  the  weight  of 
a  floating  body  is  equal  to  the  weight  of  the  fluid  which  it  displaces. 

Nicholson's  Hydrometer  is  so  contrived  as  to  determine  the  specific 
gravity  of  solids  as  well  as  liquids.  In  Fig.  31,  B  is  a 
hollow  ball,  to  which  is  attached  a  fine  wire  s,  supporting 
a  dish  C  for  receiving  weights ;  proceeding  from  the  under 
side  of  the  ball  is  the  stirrup  D,  carrying  a  heavy  dish  F 
for  preserving  the  stability  of  the  instrument  when  it 
floats,  and  for  holding  any  solid  body  whose  specific  grav- 
ity is  to  be  determined.  The  instrument  is  floated  in  pure 
water,  and  a  weight  of  1000  grains  is  put  into  the  dish  C  ; 
now,  the  weight  of  the  instrument  is  so  adjusted  that  it 
sinks  to  about  the  middle  of  the  fine  stem ;  and  a  mark  s 


is  made  at  this  point.  Fig.  31. 

31.  To  determine  the  specific  gravity  of  a  liquid:  — 
Place  the  instrument  in  the  liquid,  and  put  weights  into  the  dish  C 
until  the  mark  s  on  the  stem  sinks  to  the  level  of  the  surface  of  the 
liquid.  These  weights  added  to  the  weight  of  the  instrument  will  be 
equal  to  the  weight  of  the  liquid  displaced ;  but  the  weight  of  the  in- 
strument added  to  1000  oz.  is  equal  to  the  weight  of  an  equal  bulk  of 
water  ;  therefore  the  former  sum  divided  by  the  latter  will  give  the  spe- 
cific gravity  of  the  liquid.  For  example,  let  the  weight  of  the  instru- 
ment be  3000  grains,  the  weight  put  on  the  dish  C  equal  to  200  grains, 
then  we  have 

8 


86  NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


Weight  of  displaced  water  =  3000  -f-  1000  =  4000  ; 
"         "         "        liquid  =  3000  +    200  =  3200; 

3200 
Specific  gravity  of  liquid  =          =  .8. 


32.  To  determine  the  specific  gravity  of  a  solid  :  — 

Place  the  instrument  in  water,  and  put  the  solid  in  the  upper  dish  C  ; 
add  weights  to  this  dish  until  the  mark  s  on  the  stem  sinks  to  a  level 
with  the  fluid  ;  then  these  weights,  together  with  the  weight  of  the  body, 
must  be  equal  to  1000  grains  ;  therefore  the  weight  of  the  body  itself 
must  be  equal  to  1000  grains  less  by  these  weights.  For  example,  if  600 
grains  are  added  to  the  dish  C,  then  the  weight  of  the  body  is  equal  to 
1000  grains  less  by  600  grains,  or  400  grains. 

Let  the  body  be  now  placed  in  the  lower  dish  F,  and,  as  before,  let 
weights  be  placed  in  the  upper  dish  until  the  mark  s  sinks  to  a  level 
with  the  water;  then  these  weights,  together  with  the  weight  or  down- 
ward tendency  of  the  body  in  the  water  are  equal  to  1000  grains  ;  there- 
fore.the  weight  of  the  body  in  \vater  is  equal  to  1000  grains  less  by  the 
weights  added  to  the  upper  dish.  Suppose  these  weights  to  make  up 
800  grains  ;  then  the  weight  of  the  b.ody  in  water  is  equal  to  1000  grains 
less  by  800  grains,  or  200  grains. 

Now,  having  obtained  the  weight  of  the  body,  400  grains,  and  also 
its  weight  in  water,  200  grains,  the  loss  of  weight  in  water  will  be  equal 
to  the  difference  of  these  weights  —  that  is,  in  this  case,  the  loss  of  weight 
in  water  will  be  equal  to  400  grains  less  by  200  grains,  or  200  grains  ; 
hence  we  have,  by  Art.  27,  — 

wt.  body.  400 

Specific  gravity  of  the  body  =  -  :  -  :  --  =  ——  =  2. 
wt.  lost  in  water       200 

Let  us  take  another  example.  In  finding  the  weight  of  the  body, 
suppose  that  300  grains  were  put  into  the  dish  ;  and  in  finding  the  weight 
of  the  body  in  water,  suppose  that  400  grains  were  put  into  the  dish  ; 
then  we  have,  — 

Weight  of  the  body  =  1000  —  300  =  700  ; 

Weight  of  body  in  water  =  1000  —  400  =  600  ; 

Weight  lost  in  water  =  700  —  600  =  100  ; 

700 
Specific  gravity  of  the  body  =  —  —  =  7. 

33.  Sike's  Hydrometer,  which  is  the  one  employed  by  excisemen,  has  a 
graduated  stem,  and  the  instrument  is  always  used  in  connection  with  a 
book  of  tables.     The  depth  to  which  the  stem  sinks  is  observed,  and  at 
the  same  time  the  thermometer  and  barometer  are  also  noted;  these 
numbers  being  sought  out  in  the  tables,  the  corresponding  specific  grav- 
ity is  found  in  its  proper  column. 

The  hydrometer  is  chiefly  used  for  ascertaining  the  adulteration  of 


^HYDROSTATICS   AND   HYDRAULICS.  87 

spirits.  The  strongest  spirits,  or  those  which  contain  the  largest  quantity 
of  alcohol,  have  the  least  specific  gravity,  and  consequently  the  hydrom- 
eter sinks  in  them  to  the  greatest  depth.  The  specific  gravity  of  pure 
alcohol  is  nearly  T8<y  or  .8,  and  that  of  proof  spirits,  which  is  a  mixture 
of  equal  parts  of  alcohol  and  water,  is  about  T9(j-  or  .9.  Spirits  are  said 
to  be  above  proof  or  under  proof t  according  as  they  contain'  a  larger  or 
smaller  proportion  of  alcohol. 

Floating  Bodies. 

34.  It  has  already  been  explained  that,  when  a  body  floats  in  a  fluid, 
the  weight  of  the  fluid  displaced  is  always  equal  to  the  weight  of  the 
body.     Let  A  B  C  D  (Fig.  32)  be  a  piece  of 

wood  floating  in  water  ;  then  the  weight  of  the 
water  displaced,  viz.,  E  F  C  D,  is  equal  to  the 
whole  weight  of  the  wood.  The  upward  pres- 
sure on  the  bottom  D  C  is  the  same  as  that  which 
would  support  a  portion  of  fluid  equal  hi  bulk  to 
the  displaced  fluid  E  F  C  D  ;  and  as  the  down-  Fig.  32. 

ward  pressure  of  the  body  is  equal  to  the  upward 

pressure  of  the  fluid,  it  follows  that  the  weight  of  the  body  is  equal  to 
the  weight  of  the  fluid  displaced. 

Hence  it  is  that  iron  vessels  float  in  water ;  for  as  they  are  made  hol- 
low, it  is  easy  to  see  that  the  displaced  water  must  be  much  heavier  than 
the  whole  weight  of  the  metal. 

35.  In  order  that  a  body  may  float  with  stability,  it  is  requisite  that 
its  centre  of  gravity  should  lie  as  low  as  possible.     For  this  reason  ballast 
is  laid  in  the  bottoms  of  ships  ;  and,  in  like  manner,  when  a  boat  is  in 
danger  of  being  overturned  by  the  violence  of  the  winds  or  the  rolling 
of  the  waves,  it  tends  to  lessen  the  danger  when  the  passengers  lay 
themselves  flat  at  the  bottom  of  the  boat.     A  body  is  most  stable  when 
it  floats  upon  its  greatest  surface ;  thus  a  plank  floats  with  the  greatest 
stability  when  it  is  placed  flat  upon  the  water,  and  its  position  is  unstable 
when  it  is  made  to  float  edgewise.     A  body  will  only  remain  at  rest  in  a 
fluid  when  the  centres  of  gravity  of  the  whole  body  and  that  of  the  dis- 
placed Jiuid  are  in  the  same  vertical  line  ;  for  if  the  body  is  shifted  from 
this  position,  the  upward  pressure  of  the  water,  as  well  as  the  downward 
pressure  of  the  body,  tends  to  bring  it  to  its  original  position.  In  Fig.  33, 
No.  1,  C  represents  the  centre 

of  gravity  of  the  body,  and  B 
that  of  the  fluid  displaced, 
where  C  and  B  are  in  the  same 
vertical  line.  Now,  when  the 
body  is  shifted  from  this  posi- 

Fig.  33. 


C3  NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

tion,  as  in  No.  2,  the  gravity  of  the  body,  as  well  as  the  buoyancy  of 
the  fluid,  tends  to  bring  the  body  to  its  first  position. 

ADDITIONAL  FACTS. 

A  stone  which  on  land  requires  the  strength  of  two  men  to  lift 
it  may  be  lifted  in  water  by  one  man.  A  boy  will  often  wonder 
why  he  can  lift  a  certain  stone  to  the  surface  of  the  water,  but  no 
further. 

When  a  person  lies  in  a  bath,  the  limbs  are  so  nearly  supported  by 
the  water  as  to  require  scarcely  any  exertion  on  the  part  of  the  indi- 
vidual. 

The  human  body,  with  the  chest  full  of  air,  naturally  floats  with  a 
bulk  of  about  half  the  head  above  the  water.  That  a  person  in  water, 
therefore,  may  live  and  breathe,  it  is  only  necessary  to  keep  the  face  up- 
permost. 

The  common  contrivances  called  life  preservers,  for  preventing  drown- 
ing, are  strings  of  corks  put  round  the  chest  or  neck,  or  air-tight  bags, 
inflated,  and  applied  round  the  upper  part  of  the  body. 

Fishes  can  change  their  specific  gravity  by  diminishing  or  increasing 
the  size  of  a  little  air  bag  contained  in  their  bodies. 

A  ship  draws  less  water,  or  sails  lighter,  by  one  thirty-fifth,  in  the 
heavy  salt  water  of  the  sea  than  in  the  fresh  water  of  a  river ;  and,  for 
the  same  reason,  swimming  in  sea  water  is  easier  than  in  a  pond  or  river. 

Many  kinds  of  wood  that  float  in  water  will  sink  in  oil. 

A  man  floats  on  mercury  as  the  lightest  cork  floats  on  water. 

Cream  rises  in  milk,  and  forms  a  covering  to  it. 

The  equilibrium  of  floating  bodies  is  a  subject  of  great  practical  im- 
portance, but  it  would  require  a  knowledge  of  mathematics  to  enter 
upon  it  more  fully. 

Capillary  Attraction. 

36.  When  the  extremity  of  a  glass  tube  having  a  very  small  bore  is 
plunged  into  water,  the  fluid  is  found  to  rise  in  the 
tube.    This  exception  to  the  law  of  level  of  the  surface 
of  a  fluid  is  said  to  take  place  in  consequence  of  the 
attraction  of  the  interior  surface  of  the  tube  upon  the 
water ;  and  it  is  called  capillary  attraction,  for  it  takes 
place  in  capillary  tubes,  or  tubes  having  a  hair-like 
bore.    The  adhesion  of  the  water  to  the  sides  of  the 
tube  is  shown  by  the  concave  form  of  the  surface  of  the         Fig.  34. 
water  in  the  tube ;  hence  it  is  always  essential  to  the 


HYDROSTATICS    AND    HYDRAULICS. 


89 


effect  that  the  tube  should  be  susceptible  of  being  wet- 
ted, for  when  the  tube  is  soiled  with  any  oily  sub- 
stances the  water  be'comes  depressed  in  consequence  of 
the  repulsion  which  the  oil  has  for  water.  If  the  cap- 
illary tube  be  immersed  in  mercury,  then  the  mercury 
becomes  depressed  in  the  tube,  in  consequence  of  the 
repulsion  which  the  surface  of  the  glass  has  for  the 
mercury. 


Fig.  35. 


37.  The  height  to  which  water  rises  in  these  tubes  is  in  proportion  to 
the  smallness  of  their  diameters ;   thus  in  two  tubes,  one  of  which  is 
double  the  diameter  of  the  other,  the  fluid  will  rise  to  double  the  height 
in  the  small  tube  that  it  will  do  in  the  other.     This  law  is  beautifully 
illustrated  by  the  following  experiment  (Fig. 

36)  :  Take  two  plates  of  glass,  kept  in  contact 
at  one  extremity  and  a  little  apart  at  the 
other ;  immerse  them  in  water,  as  shown  in 
the  figure  :  the  water  rises  between  the  plates, 
forming  a  curved  line  called  the  hyperbola. 
It  will  be  observed  that  the  height  of  the 
water  at  any  part  is  greater  according  as  the 
distance  between  the  plates  at  that  part  is  less. 

38.  If  two  balls  of  wood,  (Fig.  37,)  each  of  which  is  capable  of  be- 
coming wetted,  be  placed  upon  water,  their 

sides  will  draw  up  the  water ;  and  if  they  are 
brought  near  one  another,  so  that  the  eleva- 
tions of  the  fluid  may  interfere,  the  balls  will 
approach  one  another  —  that  is,  they  will  ap- 
pear to  attract  one  another.    In  the  same  way 
little  floating  bodies  are  attracted  to  the  sides  of  the  wood.     If  one  of 
the  balls  be  soiled  with  oil,  (Fig.  38,)  the  fluid  about  that  ball  will  be 
depressed ;  and  if  they  are  brought  near  one 
another,  as  in  the  last  case,  they  will  repel  one 
another.    These  facts  depend  upon  the  princi- 
ple of  capillary  attraction  and  repulsion.     On 
the  same  principle  a  great  many  phenomena 
in  nature  may  be  explained.     For  example, 
the  melted  tallow  of  a  candle  rises  in  the  wick  ;  and  water  rises  through 
the  fine  pores  of  sugar. 

39.  Take  an  ordinary  sized  glass  tube,  and  tie  a  piece  of  thin  bladder, 
or  any  fine  membranous  substance,  over  one  end ;  into  this  tube  pour 
some  thick  sirup  of.  sugar  and  water ;  immerse  the  tube  in  water ;  then 
in  the  course  of  a  few  hours  the  fluid  in  the  tube  will  have  risen  to  the 

8* 


Fig.  37. 


Fig.  38. 


90 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


height  of  several  inches  above  the  level  of  the  water  in  the  vessel.  Here 
it  appears  that  the  thinner  fluid,  the  water,  passes  more  rapidly  through 
the  pores  of  the  bladder  into  the  tube  than  the  thicker 
fluid,  the  sirup,  passes  out  of  it.  This  remarkable  phenom- 
enon is  called  Endosmose  and  Exosmose,  the  former  term 
meaning  to  tend  inwards,  the  latter  to  tend  outtcards.  In 
the  foregoing  experiment  the  water  passes  through  the 
pores  of  the  bladder  into  the  tube  by  endosmose,  and  the 
thicker  fluid  passes  out  of  the  tube  by  exosmose.  On 
this  principle  a  great  many  important  phenomena  of  nature 
may  be  explained.  pig.  39. 


HYDRAULICS. 

40.  Having  explained  the  leading  phenomena  resulting  from  the 
pressure  and  weight  of  fluids  in  a  state  of  rest,  we  now  come  to  treat  of 
the  motion  of  fluids. 


Velocity  with  which  Water  spouts  out  of  a  Vessel. 

41.  "When  a  hole  is  made  hi  a  vessel  filled  with  water,  the  fluid  spouts 
out  in  a  jet  with  greater  or  less  velocity  according  to  the  depth  of  the 
hole  below  the  surface  of  water.  The  following  simple  law  obtains  in 
reference  to  the  efflux  of  the  water,  supposing  that  it  underwent  no  resist- 
ance from  friction  or  other  causes.  The  velocity  of  a  jet  B  or  C  (Fig. 
40)  proceeding  vertically  from  a  vessel  is  such  as 
to  cause  the  water  to  rise  up  to  the  level  of  the 
water  in  the  vessel,  as  shown  in  the  annexed  cut. 
This  seems  to  arise  from  the  principle  that  water 
always  seeks  its  level,  for  the  jet  tends  to  rise  to 
the  level  A  D  of  the  water  in  the  vessel.  Now, 
if  the  velocity  with  which  the  fluid  issues  from 
the  aperture  B  be  such  as  to  carry  the  fluid 
through  the  perpendicular  height  B  A  in  opposi- 


Fig.  40. 


tion  to  gravity,  it  follows  that  this  velocity  is  equal  to  that  which  a 
body  would  acquire  in  falling  freely  through  this  space.  Hence  we  con- 
clude that  a  fluid  issues  from  an  aperture  with  a  velocity  equal  to  that 
which  a  body  would  acquire  in  fatting  through  a  space  equal  to  the 
depth  of  the  aperture  beloio  the  surface  of  the  fluid :  thus,  if  A  B  is  16 
feet,  the  velocity  of  the  jet  will  be  32  feet  per  second ;  for  this  is  the 
velocity  which  a  body  acquires  in  falling  through  the  space  of  16  feet. 


HYDROSTATICS    AND    HYDRAULICS. 


Fig.  41. 


42.  In  Fig.  41  the  aperture  is  made  in  the  bottom  of 
the  vessel ;  and  the  theoretical  velocity  with  which  the 
water  issues  is,  as  in  the  preceding  case,  equal  to  the  ve- 
locity which  the  fluid  would  acquire  in  falling  freely 
down  from  m  n  to  b  c. 

Now,  it  is  shown  in  mechanics  that  the  velocity  ac- 
quired by  a  falling  body  is  as  the  square  root  of  the  i 
space  through  which  it  falls ;  therefore  the  velocity  with 
which  water  spouts  out  at  any  aperture  in  a  vessel  is  as 
the  square  root  of  the  depth  of  the  aperture  below  the  sur- 
face of  the  water.     It  must,  however,  be  observed  that 
there  are  different  obstructions  which  tend  to  modify  this  rule  in  practice. 
When  water  is  conveyed  from  a  cistern  to  any  considerable  distance  in 
pipes,  as  shown  in  the  annexed  cut, 
(Fig.  42,)  the  friction  of  the  water, 
as  it  moves  in  the  pipe,  together  with 
the  obstructions    presented    by  the 
ben  dings,  &c.,  tends  very  much  to 
retard  the  motion  of  the  fluid.     By  pig,  42. 

the  theoretical  rule  above  given,  the 

velocity  of  discharge  would  be  due  to  the  vertical  depth  A  B  through 
which  the  water  falls ;  but,  owing  to  the  resistances  just  mentioned,  this 
is  very  far  from  being  practically  true ;  in  such  cases  the  engineer  must 
have  recourse  to  some  formula  derived  from  experiment. 

It  is  a  curious  fact  that  more  water  issues  from  a  vessel  through 
a  short  pipe  than  through  a  simple  aperture  of  the  same  diameter  as  the 
pipe ;  and  still  more,  if  the  pipe  be  funnel-shaped,  or  wider  towards  its 
inner  extremity.  The  explanation  is,  that  the  issuing  particles,  coming 
from  all  sides  to  escape,  cross  and  impede  each  other  in  rushing  through 
a  simple  opening,  whereas  the  tube,  leading  the  water  by  a  more  grad- 
ual inclination  towards  the  point  of  exit,  considerably  prevents  the  cross- 
ing among  the  particles. 

To  regulate  the  Supply  of  Water. 

43.  "When  water  is  conveyed  by  pipes  to  cisterns,  it  is  necessary  that 
no  more  water  should  flow  into  the  cistern  than  is  required.     This  ad- 
justment is  effected  by  a  simple  and  ingenious  contrivance  called  the 
float  cock.     Fig.  43,  P  represents  a  pipe 

conveying  water  to  the  cistern  A ;  B  is 
a  hollow  ball  of  metal,  called  the  float, 
which  is  connected  with  a  cock  C,  open- 
ing and  closing  the  pipe  in  such  a  man- 
ner that  when  the  float  is  raised  the  cock 

**  Fig.  43. 


^  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

stops  the  passage  of  the  water,  and,  on  the  contrary,  when  the  float  is 
depressed,  the  cock  allows  the  water  to  flow  through  it.  Now,  when 
there  is  a  deficiency  of  water  in  the  cistern,  the  cock  C  is  open  and  a 
fresh  supply  is  allowed  to  run  in ;  but  as  the  water  rises,  the  float  B  by 
its  buoyancy  also  rises,  and  at  length  turns  the  cock  so  as  to  stop  the 
supply  of  water ;  again,  when  water  is  taken  out  of  the  cistern,  the  float 
falls  with  the  water,  and  at  length  opens  the  cock,  which  admits  a  fresh 
supply  of  water ;  and  so  on. 

Springs  and  Artesian    Wells. 

44.  Springs  are  formed  by  the  rain  and  moisture  which  fall  upon 
hills  and  mountains.     The  upper  crust  of  mountains  is  usually  composed 
of  loose,  porous  layers  of  substances  which  allow  water  to  pass  through 
them,  and  also  of  layers  of  clay  and  solid  substances  which  are  imper- 
vious to  water.     Let  the  accompanying 

cut  (Fig.  44)  represent  the  section  of  a 
mountain  or  hill,  where  A  is  composed  of 
loose  or  porous  substances,  c  a  layer  of  clay 
or  some  substance  which  stops  the  descent 
of  the  water ;  then  the  water  which  filters 
thimigh  A  will  run  along  the  top  of  c 
until  it  is  discharged  at  F  in  the  form  of  Fig.  44. 

a  natural  spring  or  fountain  ;  B  is  com- 
posed of  loose  or  porous  substances ;  D  some  substance  which  stops  the 
descent  of  the  water ;  w  is  an  artesian  well,  or  vertical  hole,  which  has 
been  bored  by  wrorkmen,  and  metal  pipes  put  down  it ;  now,  the  rain 
water,  together  with  the  water  which  arises  from  melting  snow  and  ice, 
sinks  through  B  and  flows  along  the  surface  of  D  until  it  finds  a  vent 
up  the  pipes  forming  the  artesian  well  w.  Let  us  further  suppose  that  b 
is  a  rent  or  fissure  in  which  water  is  collected  ;  then  the  height  to  which 
the  water  will  rise  in  the  well  w  will  be  on  the  same  level  with  the  water 
in  the  fountain  b. 

Canals  and  Locks. 

45.  Canals  are  artificial  streams  of  water,  upon  which  barges  are 
floated  for  the  purpose  of  conveying  heavy  goods  from  one  place  to 
another.    The  water  in  canals  is  usually  obtained  from  springs  or  from 
some  neighboring  river.     In  order  that  the  barges  may  sail  with  equal 
ease  in  both  directions  of  the  canal,  it  is  requisite  that  the  surface  of  the 
water  should  be  level ;  to  accomplish  this,  the  canal  is  sometimes  carried 
over  valleys  by  means  of  bridges  and  embankments,  and  sometimes  it  is 
even  made  to  pass  through  hills  by  means  of  tunnels ;  but  the  most 


HYDROSTATICS    AND    HYDRAULICS. 


93 


Fig.  45. 


common  contrivance  for  maintaining  the  level  of  surface  is  that  of  locks 
or  floodgates.     Pig.  45   repre- 
sents a  section  of  a  lock,  made      ^=77     n       PA  c 
at  a  place  where  there  is  a  sud- 
den fall  of  the  ground  along 
which  the  canal  has  to  pass : 
A  B  and  C  D  are  the  two  gates 
which  completely  intercept  the 
course  of  the  water,  but  at  the 

same  time  admit  of  being  opened  and  closed ;  A  H  is  the  level  of  the 
water  in  that  part  of  the  canal  lying  above  the  gate  A  B,  and  F  G  the 
level  lying  below  the  gate  C  D ;  now,  when  a  barge  is  about  to  pass 
from  A  H  to  F  G,  a  side  sluice,  not  shown  in  the  figure,  is  first  opened, 
which  allows  the  water  to  flow  from  A  H  into  the  space  A  E  F  C  be- 
tween the  gates  until  it  attains  the  common  level  II A  C ;  the  gate  A  B 
is  then  opened,  and  the  barge  floats  into  the  space  between  the  gates ; 
the  gate  A  B  is  now  closed,  and  a  side  sluice  is  opened,  which  allows  the 
water  to  flow  from  the  space  A  E  F  C  until  it  comes  to  the  common  level 
E  F  G ;  the  gate  C  D  is  then  opened,  and  the  barge  floats  out  of  the 
locks  along  the  canal.  It  is  easy  to  see,  by  reversing  the  steps  of  this 
process,  that  the  barge  may  be  floated  in  the  contrary  direction.  A 
horse,  moving  along  the  side  of  the  canal,  is  usually  employed  to  pull 
the  barge  through  the  water. 


HYDRAULIC    MACHINES. 


Water  Wheels, 


46.  Fig.  46  represents  an 
undershot  wheel,  turning  on. 
the  axle  A;  M  N  is  a  cur- 
rent of  water,  which,  striking 
against  the  float  boards,  causes 
the  wheel  to  revolve  on  its 
axle  A  ;  on  this  axle  is  fixed 
the  toothed  wheel  which  drives 
the  machinery. 

In  Poncelet's  undershot 
wheel  the  float  boards  are 
curved  towards  the  direction 
of  the  current,  so  that  the 
water  rolls  up  their  surface, 


Fig.  46. 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


and  does  not  leave  them  until 
all  its  work  is  spent  upon  the 
wheel. 

Fig.  47  represents  an  overshot 
wheel,  turning  on  its  centre  O  ; 
E  F  is  a  stream  of  water  flow- 
ing over  the  top  of  the  wheel 
into  the  buckets  c  d,  &c.,  fixed 
upon  the  rim  of  the  wheel ;  the 
gravity  of  the  water  in  these 
buckets  causes  the  right  hand 
side  A  B  C  of  the  wheel  to  be 
heavier  than  the  other  side, 
where  the  buckets  are  empty, 
being  all  turned  upside  down  ; 
hence  the  wheel  revolves  in  the 
direction  ABC.  Let  c  m  and  d  n  be  perpendiculars  let  fall  from  the 
centres  of  gravity  of  the  water  in  the  two  buckets  c  and  d  respectively  ; 
then  O  m  will  be  the  leverage  of  the  water  in  the  bucket  c,  and  O  n 
that  of  the  bucket  d ;  the  bucket  B,  in  the  horizontal  line  O  B,  will 
have  the  greatest  leverage,  and  consequently  will  act  with  the  greatest 
efficiency  in  moving  the  wheel.  As  the  buckets  descend  below  B,  they 
not  only  act  with  a  decreasing  leverage,  but  the  water  which  they  con- 
tain is  continually  flowing  out  of  them  until  they  arrive  at  C,  when  they 
become  completely  empty. 

Barker's  Mill 

47.  This  simple  and  elegant  engine 
is  moved  by  the  efflux  of  water  under- 
going pressure.  C  D  is  a  hollow  cylin- 
der turning  on  a  vertical  axis ;  A  B  is 
a  horizontal  cylinder  communicating 
internally  with  the  former  ;  ar  the  ex- 
tremities of  this  horizontal  cylinder  two 
apertures  A  and  B  are  made  in  the 
sides,  opening  in  opposite  directions. 
On  the  continuation  of  the  vertical 
axis,  the  upper  millstone  S  is  fixed,  and 
therefore  revolves  with  it ;  H  is  the 
hopper  delivering  the  corn  to  be  ground. 
A 'continuous  stream  of  water  flows 
through  the  pipe  e  c  into  the  cylinder 
C  D.  Let  us  suppose  that  the  cylinder 


Fig.  28. 


HYDROSTATICS    AND    HYDRAULICS.  95 

C  D,  with  its  horizontal  branch  A  B,  to  be  filled  with  water ;  then  the 
pressure  of  this  column  of  fluid  will  cause  the  water  to  be  projected  in 
jets  from  the  orifices  A  and  B  in  opposite  directions  ;  then  the  recoil  or 
reaction  of  these  jets  upon  the  extremities  of  A  and  B  gives  a  rotatory 
motion  to  the  whole  machine  upon  the  vertical  axis. 

Or,  to  take  another  view  of  the  principle  of  action  in  this  machine : 
if  the  orifices  at  A  and  B  were  closed,  the  column  of  fluid  in  the  vertical 
tube  C  D  would  press  eqiially  on  both  sides  of  the  horizontal  tube  A  B  ; 
but  when  the  orifices  A  and  B  are  opened,  the  pressure  on  these  parts  is 
released,  while  the  pressure  upon  the  sides  opposite  to  them  remains  the 
same ;  hence  the  tube  A  B  revolves  in  the  direction  of  the  greater 
pressure  —  that  is,  in  a  direction  contrary  to  that  of  the  jets  of  water. 


The  Archimedean  Screw. 

48.  This  simple  and  beautiful  contrivance  for  raising  water  was  in- 
vented by  the  great  Archimedes.  It  simply  consists  of  a  pipe  wound, 
in  a  spiral  form,  about  a  solid  cyl- 
inder A  B,  which  is  made  to  revolve 
on  its  axis  by  means  of  the  winch 
H.  The  lower  orifice  a  of  the 
spiral  tube  dips  into  the  water  to 
be  raised,  and  it  is  discharged  at 
the  upper  orifice.  As  the  cylinder 
is  turned  round,  the  water,  which 
enters  the  orifice  a  at  each  revolu- 
tion, runs  down  a  series  of  inclined 
planes,  until  it  flows  out  at  the 
upper  orifice.  In  order  to  illustrate 
this  action,  let  a  marble  be  put  into 
the  pipe  at  a,  then  as  the  cylinder 
is  turned  round,  the  marble  will  continue  to  roll  down  a  succession  of 
inclined  planes  (formed  at  each  revolution  of  the  cylinder)  until  it  is 
discharged  at  the  upper  orifice. 


EXERCISES  ON  HYDROSTATICS  AND  HYDRAULICS. 

1.  In  Fig.  9,  Art.  11,  suppose  the  large  piston  P  to  contain  40  square 
inches,  and  the  small  one,  pt  2  square  inches ;  what  upward  pressure 
will  be  produced  upon  the  large  piston  by  a  downward  pressure  of  H 
Ibs.  exerted  upon  the  small  one  ? 


yb     .       NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Pressure  on  2  in.  of  the  piston  =  14  Ibs. ; 

«  1  in.  "        =  U:  =  7  Ibs. ; 

«          40  in.  "        =  40  X  7  =  280  Ibs. 

2.  Required  the  same  as  in  the  last  example,  when  the  surface  of  the 
large  piston  is  25  inches,  that  of  the  small  one  3  inches,  and  the  pressure 
applied  to  it  30  Ibs.  Am.  250  Ibs. 

3.  In  Fig.  11,  Art.  13,  let  the  area  of  the  base  D  C  contain  4  square 
feet,  and  let  the  depth  of  the  water  G  C  be  5  feet :  required  the  pressure 
on  the  bottom  of  the  vessel.  Ans.  20,000  oz.,  or  1250  Ibs. 

4.  In  the  hydrostatic  bellows,  (see  Fig.  18,  Art.  16,)  the  upper  board 
A  contains  2  square  feet  of  surface,  and  the  height  of  the  water  in  the 
tube  b  c  is  4  feet :  required  the  weight  W  which  will  be  supported  on  the 
bellows.  Ans.  8000  oz.,  or  500  Ibs. 

5.  In  a  flood  gate  (see  Fig.  23,  Art.  21)  A  Q  D  G,  let  the  breadth 
A  Q  =  5  feet,  the  depth  A  G  or  E  F  =  6  feet :  required  the  position  of 
the  centre  of  pressure,  and  also  the  pressure  of  the  water  upon  the  gate. 

Ans.  The  centre  of  pressure  is  two  feet  from  the  bottom ;  and  the 
whole  pressure  is  5625  Ibs. 

6.  In  finding  the  specific  gravity  of  a  liquid,  (see  Art.  26,)  suppose 
the  weight  of  the  empty  bottle  to  be  300  grains,  the  weight  of  the  bottle 
filled  with  water  to  be  900  grains,  and  the  weight  of  the  bottle  filled 
with  the  liquid  to  be  700  grains :  required  the  specific  gravity  of  the 
liquid.  Ans.  f ,  or  666  -f-. 

7.  The  weight  of  a  solid  body  is  300  grains,  but  its  weight  in  water  is 
250  grains  :  required  the  specific  gravity  of  the  body.     (See  Art.  27.) 

Ans.  6. 

8.  A  solid  body  lost  40  grains  when  weighed  in  water,  and  70  grains 
when  weighed  in  oil  of  vitriol :  required  the  specific  gravity  of  the 
vitriol.     (See  Art.  28.)  Ans.  If. 

9.  In  finding  the  specific  gravity  of  a  liquid  by  Nicholson's  Hydrom- 
eter, (see  Art.  30,)  let  the  weight  of  the  instrument  be  3000  grains,  and 
let  the  weight  put  in  the  dish  C  (to  sink  the  instrument  to  the  mark  s 
when  floated  in  the  liquid)  be  1400  grains :  required  the  specific  gravity 
of  the  liquid.  Ans.  1.1. 

10.  A  cubical  piece  of  wood,  whose  side  is  2  feet,  sinks  to  the  depth 
of  1^  feet  when  floated  on  water :  required  the  specific  gravity  of  the 
woodj     (See  Art.  34.) 

Here  the  wood  contains  8  cubic  feet ;  and  the  volume  of  the  water  dis- 
placed =  2X2X1^  =  6  cubic  feet.     Now,  the  weight  of  this  displaced 
water  is  6000  oz.,  but  this  is  also  the  weight  of  the  floating  body  ; 
Weight  of  8  c.  ft.  of  the  wood  =  6000  oz. ; 

«          1  c.  ft.  «          =  6000 

-—  =  750oz.; 


HYDROSTATICS    AND    HYDRAULICS.  97 

But  the  weight  of  1  c.  ft.  of  water  is  1000  oz. ; 

Specific  gravity  of  the  wood  ==_^L=.7o. 

1000 

This  result  might  at  once  be  obtained  by  dividing  the  depth  of  im- 
mersion by  the  whole  depth  of  the  body  :  thus,  J.J  =  |,  or  .75. 

11.  Required  the  same  as  in  the  last  example,  when  the  side  of  the 
cube  is  1  foot,  and  the  depth  of  immersion  8  inches.  Ans.  f . 

12.  With  what  velocity  will  water  issue  from  an  orifice  made  at  the 
depth  of  4  feet  below  the  level  of  the  fluid  ?     (See  Art.  42.) 

Ans.  16  ft.  per  second. 

Here  it  will  be  observed  that  a  body  will  fall  through  4  ft.  in  £  of  a 
second. 

13.  Required  the  same  as  in  the  last  example,  when  the  orifice  is  64 
feet  below  the  level  of  the  fluid.  Ans.  64  feet  per  second. 

9 


PNEUMATICS. 

1.  PNEUMATICS  is  that  part  of  Natural  Philosophy  which 
treats  of  the  motion  and  pressure  of  aeriform  or  elastic  fluids, 
such  as  the  air  which  forms  the  atmosphere. 

2.  The  atmosphere  every  where  surrounds  the  globe,  and 
extends  to  the  height  of  about  fifty  miles  above  the  tops  of 
our  highest  mountains.     Although  the  air  is  invisible,  and 
seems  as  nothing  to  the  vulgar  eye,  yet  it  is  a  material  sub- 
stance, possessing  all  the  essential  properties  of  matter  in 
common  with  solid  and  liquid  bodies. 

3.  Air  retards  the  motion  of  bodies. 

Thus,  when  a  flat  board  is  rapidly  moved  through  the  air,  a  consider- 
able resisting  force  is  felt ;  and  it  is  well  known  that  the  velocity  of 
railway  trains  is  much  affected  by  the  resistance  of  the  air.  Winds,  air 
in  motion,  drive  our  ships  through  the  ocean,  and  perform  useful  labor 
in  our  wind  mills.  The  air,  driven  on  with  terrific  violence  by  the  hur- 
ricane or  the  tornado,  sweeps  over  the  earth  and  carries  desolation  and 
ruin  to  the  abodes  of  man.  The  air,  in  the  storm  and  tempest,  lifts  up 
the  mountain  billows  of  the  deep,  and  dashes  in  pieces  the  stately  bark 
as  she  bears  to  our  shores  the  wealth  of  other  lands.  It  is  plain  that  the 
agent  which  is  capable  of  producing  such  effects  must  be  material. 

4.  The  air,  like  all  material  bodies,  is  impenetrable  ;  that  is 
to  say,  the  space  occupied  by  air  cannot  contain  any  other 
body  at  the  same  time. 

EXPERIMENTS. 

Exp.  1.  Invert  a  tall  glass  A  over  water,  as  in  the 
accompanying  cut ;  the  water  does  not  rise  completely 
within  the  glass  on  account  of  the  air  which  is  in 
it.  To  render  the  experiment  more  apparent,  a  small 
cork  is  placed  upon  the  water. 

This  experiment  also  shows  the  elasticity  of  the  air ; 
for  as  the  glass  is  pressed  down,  the  air  that  is  in  it 
occupies  less  and  less  space,  and  the  force  requisite  to  pig.  1. 

(98) 


PNEUMATICS. 


99 


keep  the  glass  down,  or  to  balance  the  elastic  force  of  the  air,  increases 
-with  the  decrease  of  the  bulk  of  the  air  in  the  glass. 

Exp.  2.  Fill  a  large  bottle  with  water  ; 
blow  air  into  the  bottle  by  means  of  a 
bent  tube,  as  shown  in  the  figure ;  in 
this  case,  the  air  displaces  the  water. 

In  like  manner,  air  may  be  trans- 
ferred from  one  vessel  to  another.  Here 
b  (Fig.  3)  represents  a  vessel  filled  with 
water,  and  having  its  open  mouth  invert-  Fig.  2. 

ed  in  the  same  fluid  ;  e  is  another  vessel  containing  air  ;  the  lower  edge 
of  e  is  brought  to  the  mouth  of  b,  and  as  the  upper 
end  of  e  is  depressed,  the  air  rises  in  bubbles  into 
the  vessel  6,  and  displaces  the  water  ;  thus  all  the 
air  in  the  vessel  e  may  be  transferred,  without  any 
loss,  into  the  vessel  6. 

It  will  be  hereafter  explained,  that  the  water  is 
sustained  in  b  by  the  pressure  of  the  atmosphere. 

Exp.  3.  Take  a  bent  tube  of  glass,  open  at  both 
extremities ;  place  the  fore  finger  on  the  extremity 
B,  and  pour  water  into  A  ;  the  fluid  does  not  fill  the 
branch  B  on  account  of  the  air  which  it  contains. 
Take  away  the  finger :  then  the  air  is  displaced  from 
B,  and  the  water  stands  at  the  same  level  in  both 
branches  of  the  tube. 


5.  Air  has  weight. 


Fig.  4. 


Exp.  Take  a  Florence  flask  F,  having  a  stop  cock  S  attached  to  it ; 
exhaust  the  air  from  it  by  means  of  an  exhausting  syringe,  (see  Art.  18  ;) 
weigh  the  bottle  thus  exhausted  of  air ;  open  the  cock,  and 
allow  the  external  air  to  fill  the  bottle  ;  the  scale  on  which  the 
bottle  is  placed  will  preponderate,  and  it  will  require  about 
one  pennyweight  weight  to  restore  the  balance.  This  is  the 
weight  of  the  ah-  in  the  bottle. 

Having  found  the  weight  of  any  known  bulk  of  air,  the 
weight  of  any  other  bulk  of  it  may  be  easily  determined.  For 
example,  suppose  that  the  bottle  contains  60  cubic  inches  of 
air,  and  that  its  weight  is  18  grains  :  let  it  be  required  to  find 
the  weight  of  100  cubic  inches. 

Weight  60  c.  in.  of  air  =  18  grains  ;    • 

18 
"          1  c.  in.  of  air  =  —  grains  ; 


Fig.  5. 


100  c.  in.  of  air : 


60 
18X100 

60 


30  grains. 


100          NATURAL    AND    EXPERIMENTAL    FHILOSOPHY. 

The  weight  of  100  cubic  inches  of  atmospheric  air,  at  a  mean  temper- 
ature, has  been  found  to  be  31.01  grains.  From  this  it  follows  that  a 
cubic  foot  of  air  weighs  more  than  an  ounce,  and  that  water  is  about 
eight  hundred  times  the  weight  of  an  equal  bulk  of  air. 

6.  Light  bodies  float  in  the  air  in  the  same  way  as  a  piece  of  cork 
floats  in  water :  thus  soap  bubbles,  balloons,  clouds,  and  smoke  float  in 
the  air.  Now,  when  a  body  floats  in  a  fluid,  it  is  lighter  than  that  fluid ; 
the  air,  therefore,  is  heavier,  bulk  for  bulk,  than  balloons  or  any  of  those 
bodies  which  float  in  it. 


PRESSURE    OP   THE   AIR. 

7.  The  air,  like  all  other  material  substances,  gravitates 
towards  the  earth;  from  this  it  necessarily  follows  that  the 
atmosphere  must  exert  a  pressure  upon  all  terrestrial  bodies, 
and  moreover  that  the  pressure  on  any  given  surface  must  be 
equal  to  the  weight  of  the  column  of  air  above  that  surface. 
Air,  being  a  fluid,  presses  equally  in  all  directions.  (See  HY- 
DROSTATICS, Art.  9.) 

The  fact  of  atmospheric  pressure  is  clearly  established  by  the  following 
easy  experiments :  — 

EXPERIMENTS. 

Exp.  1.  Take  a  glass  tube,  open  at  both  ends,  and  fit  a  plug  or  piston 
P  to  it,  by  wrapping  some  cotton  round  the  end  of  a  wire ;  insert  the 
lower  extremity  of  the  tube  in  water,  as  shown  in  the 
figure  ;  raise  the  piston  :  the  water  rises  in  the  tube  by 
the  pressure  of  the  atmosphere  upon  the  surface  H  R  of 
the  water  in  the  vessel. 

This  experiment  explains  the  principle  of  the  common 
syringe.  Push  the  piston  P  (Fig.  7)  to  the  bottom  of  the 
barrel ;  insert  the  nozzle  O  into  some  water,  and  then 
raise  the  piston  :  the  water  rises  into  the  syringe  by  the 
pressure  of  the  atmosphere.  When  the  piston  is  forced 
downwards,  the  water  escapes,  through  the  orifice  O,  in 
the  form  of  a  jet.  Close  the  orifice  O  with  the  finger,  and  then  raise 
the  piston  ;  a  vacuum  is  formed  beneath  the  piston. 

Exp.  2.  Close  one  end  of  a  small  tube  with  the  fore  finger,  and  then 
fijl  it  with  water  ;  invert  the  tube  so  as  not  to  spill  any  of  the  fluid :  the 
water  remains  in  the  tube.  Here  the  water  would  fall  out  of  the  tube 
by  its  weight,  if  the  upward  pressure  of  the  atmosphere  did  not  sustain 


PNEUMATICS. 


101 


it.    The  finger,  placed  upon  the  top  of  the  tube,  takes  off  the  pressure 
of  the  air  from,  the  upper  surface  of  the  water,  while  the  upward  pressure 


Fig.  7. 


Fig.  8. 


Fig.  9. 


of  the  atmosphere  upon  the  under  surface  of  the  water  sustains  the  fluid 
in  the  tube  in  opposition  to  its  gravity.  Take  away  the  finger,  and  then 
the  water  descends  by  its  own  weight  ;  for  hi  this  case,  the  air 
presses  upon  the  upper  surface  of  the  wrater,  as  well  as  upon  its 
lower  surface. 

Exp.  3.  Fill  a  very  small-necked  bottle  with  water  ;  cautiously 
invert  the  mouth  of  the  bottle  :  the  water  remains  suspended  in 
the  bottle  by  the  upward  pressure  of  the  atmosphere. 

Exp.  4.  Fill  a  wine  glass  with  water,  and  cover  the  mouth 
with  a  piece  of  paper  ;  place  the  hand  over  the  paper,  and 
invert  the  glass  ;  take  the  hand  carefully  away  :  the  water 
remains  suspended  in  the  glass  by  the  atmospheric  pressure. 

Exp.  5.  The  bent  tube  A  B  is  closed  at  the  extremity  A, 
and  open  at  B.  Fill  the  tube  with  water  or  mercury,  as 
shown  in  the  figure,  then  the  fluid  will  be  supported  in  the 
branch  A  by  the  pressure  of  the  air  on  the  surface  of  the 
fluid  at  B.  A  tube  of  this  kind,  known  by  the  name  of 
Cooper's  Tube,  is  frequently  used  in  experimental  chemistry. 

The  bird  fountain  and  the  fountain  ink  bottle  depend  upon 
the  same  principle.  In  Fig.  12,  A  represents  the  liquid  in 
the  fountain,  and  B  the  liquid  in  the  cup.  As  the  liquid  is 
taken  from  the  cup,  an  equal  portion  descends  from  the  foun- 
tain, to  supply  the  place  of  that  which  is  taken  away.  ' 

Exp.  6.  The  common  sucker  affords  a  simple  and  beautiful 
illustration  of  the  pressure  of  the  atmosphere.     Observe  that  the  wetted 
9* 


Fi3'  10. 


102 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


piece  of  leather,  or  sucker,  is  raised  in  the  middle,  by  the  string  attached 
to  that  part ;  this  forms  a  hollow  space,  or  vacuum,  between  the  central 


Fig.  12. 


Fig.  13. 


portion  of  the  sucker  and  the  stone ;  the  pressure  of  the  atmosphere, 
therefore,  presses  the  stone  upwards  against  the  sucker.  The  stone  falls 
the  moment  a  hole  is  made  in  the  central  part  of  the  sucker. 

In  this  manner  limpets  stick  with 
such  force  to  the  rocks ;  and  on  the 
same  principle  flics  walk  on  the  ceiling, 
for  they  have  the  power  of  forming 
their  little  feet  into  suckers.  Fig.  14. 

Exp.  7.  Take  a  pair  of  common  bel- 
lows ;  observe,  while  you  raise  the  top  board  h,  that  the  valve  v  opens, 
in  consequence  of  the  external  air  rushing 
in  to  fill  up  the  void ;  and  observe,  while 
you  depress  the  top  board,  that  the  valve  v  is 
closed,  and  the  air  is  propelled  through  the 
nozzle  n  with  considerable  force,  in  conse- 
quence of  the  elasticity  of  the  compressed 
air  within  the  bellows. 

Exp.  8.  Take  a  glass  tube,  about  32  inches 
long,  closed  at  one  extremity  ;  fill  the  tube 
with  mercury,  apply  the  finger  to  the  open 
end,  and  immerse  it  in  a  cup  of,  mercury ; 
bring  the  tube  to  an  erect  position,  as  shown 
in  the  accompanying  figure  :  a  column  of 
mercury  about  30  inches  high  remains  sup- 
ported in  the  tube  by  the  pressure  of  the 
atmosphere  upon  the  surface  of  the  mercury 
in  the  cup.  The  space  in  the  upper  part  of 
the  tube  is  a  vacuum. 


Fig.  15. 


PNEUMATICS.  103 

This  remarkable  experiment  was  first  made  by  Torricelli,  who  was  a 
pupil  of  the  celebrated  Galileo,  and  hence  it  has  been  called  the  Torri- 
cellian experiment. 

8.  The  average  pressure  of  the  air  is  15   Ibs.  per  square 
inch. 

The  column  of  mercury  which  balances  the  pressure  of  the  air  is  esti- 
mated from  o  to  ri,  (see  Fig.  15 ;)  that  is,  it  is  equal  to  the  height  of  the 
mercury  in  the  tube  above  the  level  of  the  mercury  in  the  cup.  As  this 
column  of  mercury  balances  the  pressure  of  the  air,  so  therefore  the  weight 
of  the  mercury  in  the  tube  is  equal  to  the  pressure  of  the  air  upon  a 
surface  equal  to  the  internal  section  of  the  tube.  (See  HYDROSTATICS, 
Art.  15.)  For  example,  let  the  internal  section  of  the  tube  be  1  square 
inch,  and  the  height  of  the  column  of  mercury  o  n  30  inches;  then  there 
will  be  30  cubic  inches  of  mercury  in  the  tube ;  now,  1  cubic  inch  of 
mercury  weighs  very  nearly  half  a  pound ;  therefore  the  weight  of  the 
mercury  in  the  tube  will  be  15  pounds ;  but  this  weight  of  mercury  bal- 
ances the  pressure  of  the  air  exerted  on  1  inch  of  surface ;  therefore  the 
pressure  of  the  air  upon  1  inch  of  surface  is  about  15  pounds. 

The  height  of  the  column  of  mercury  is  not  affected  by  the  size  of  the 
tube  ;  for  if  the  section  of  the  tube  were  2  inches,  in  the  place  of  1,  the 
weight  of  the  mercury  would  be  doubled ;  but  the  pressure  of  the  air, 
in  this  case,  would  also  be  doubled,  inasmuch  as  it  would  act  upon  2 
inches  of  surface,  in  the  place  of  1. 

The  pressure  of  the  air  will  support  a  much  longer  column  of  water 
than  of  mercury ;  for  water  being  about  13£  times  lighter  than  mercury, 
the  column  of  water  must  be  13^  times  the  length  of  the  column  of 
mercury  to  produce  the  same  amount  of  pressure.  (See  HYDROSTATICS, 
Art.  25,  Exp.  2.)  Now,  we  have  seen  that  it  takes  about  30  inches  of 
mercury  to  balance  the  pressure  of  the  air ;  therefore  it  will  take  13£ 
times  30  inches,  or  about  34  feet  of  water,  to  balance  this  pressure  ;  that 
is  to  say,  upon  an  average,  the  pressure  of  the  air  is  able  to  sustain  a 
column  of  icater  34  feet  high.  Hence  it  is  that  water  cannot  be  raised 
higher  than  34  feet  by  the  common  pump. 

9.  The  pressure  of  the  atmosphere  on  our  bodies  is  essential  to  health; 
for  it  counterbalances  the  pressure  of  the  fluids  within  us,  and  thereby 
gives  a  spring  and  elasticity  to  their  motion.     When  the  weight  of  the 
air  is  taken  away  from  any  part  of  our  bodies,  the  internal  pressure  of 
the  blood  causes  those  parts  to  swell  out ;  hence  it  is  that  persons  expe- 
rience an  unpleasant  sensation  when  they  ascend  a  high  mountain. 


104          NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


The  Barometer. 

10.  The  Torricellian  experiment  not  only  exhibits  the  principle  of  the 
barometer,  but  also  shows  the  manner  in  which  it  is  made.     It  has  been 
found  that  the  pressure  of  the  atmosphere  is  not  always  the  same ;  some- 
times it  will  support  a  column  of  mercury  equal  to  31  inches,  whereas 
at  other  times  it  will  only  support  a  column  of  28  inches.     Now,  the 
barometer  is  an  instrument  contrived  to  measure  the  weight  or  pressure 
of  the  air  at  any  time ;  in  order,  therefore,  to  enable  us  to  see  the  height 
of  the  mercury  in  the  tube,  there  is  a  scale  placed  at  the  upper  end  n 
(see  Fig.  15)  giving  the  distance  from  the  surface  of  the  mercury  in 
the  cup. 

If  a  barometer  be  taken  to  the  top  of  a  mountain,  the  mercury  in  the 
tube  will  fall ;  because,  as  we  ascend  above  the  level  of  the  sea,  the  pres- 
sure of  the  atmosphere  becomes  less  and  less.  In  this  way  the  barometer 
is  sometimes  used  to  determine  the  height  of  mountains.  It  is  also  used 
as  a  weather  gauge ;  for,  when  the  air  is  dense  and  heavy,  the  mercury 
in  the  barometer  stands  high ;  and  in  such  states  of  the  atmosphere  we 
generally  have  fine,  clear  weather ;  but,  on  the  contrary,  when  the  air 
becomes  rare  and  light,  the  mercury  in  the  barometer  falls,  and  then  we 
are  likely  to  have  rainy  or  stormy  weather. 

A  barometer  tube  is  sometimes  attached  to  air  pumps,  for  the  purpose 
of  indicating  the  degree  of  exhaustion  produced  in  the  receiver. 

The  Siphon. 

11.  This  instrument  is  used  for  drawing  off  liquids  from  vessels  which 
it  would  be  inconvenient  to  move  from  the  place  where  they  stand.     It 
simply  consists  of  a  bent  tube  B  A  C  having  one  branch  A  B  longer  than 
the  other  one  A  C. 

Experiment.  —  Fill  the  bent  tube  BAG  with  wa- 
ter ;  place  a  finger  on  B,  and  another  on  C ;  invert  the 
tube,  and  immerse  the  short  leg  in  the  water  ;•  take 
away  the  finger  :  then  the  water  imniediately  runs  in  a 
stream  from  the  orifice  B.  Hold  the  vessel  in  such  a 
position  as  to  bring  the  orifice  B  on  a  level  with  C : 
the  water  then  ceases  to  flow. 

The  principle  of  the  siphon  is  exceedingly  simple : 
the  column  of  water  A  B  being  longer,  and  of  course 
heavier,  than  the  column  A  C,  the  fluid  necessarily  Fig.  16. 

flows  in  the  direction  of  the  greater  pressure.     At  the 
same  time,  it  is  to  be  observed  that  the  pressure  of  the  atmosphere,  tend- 
ing to  force  the  water  up  the  leg  C  A,  is  the  same  as  that  which  is  tend- 


.PNEUMATICS. 


105 


ing  to  force  the  water  up  the  leg  B  A,  so  that  the  one  exactly  balances 
the  other,  and  therefore  the  water  is  left  to  descend  by  its  excess  of  grav- 
ity in  the  leg  A  B. 

Intermitting  Springs. 

12.  The  principle  of  the  siphon  enables  us  to  explain  the  nature  of 
intermitting  springs,  or  those  springs  which  only  flow  at  stated  periods. 
A  D  E  represents  a  cavity  in  a  hill, 
which  becomes  gradually  filled 
with  water  from  the  rain  and  snow 
draining  through  the  porous  earth 
or  rocks ;  A  B  C  is  a  siphon-shaped 
fissure  proceeding  from  this  cavity ; 
as  the  water  collects  in  the  cavity, 
it  rises  higher  and  higher  in  the  leg 
A  B  until  it  reaches  the  level  K  B, 
when  it  begins  to  flow  through  the 
long  leg  B  C;  and  as  the  water  Fig.  17. 

continues  to  rise  in  the  cavity,  the 

discharge  at  C  will  also  increase  until  the  water  flows  in  a  continuous  jet. 
Now,  on  the  principle  of  the  siphon,  the  water  will  continue  to  flow 
from  C  until  the  water  in  the  cavity  sinks  to  the  level  of  A  E,  when  the 
air  will  rush  into  the  siphon  ABC;  and  then  the  water  will  not  flow 
again  until  it  has  reached  the  level  K  B,  so  that  the  spring  will  appear 
to  have  regular  intervals  of  repose. 


ELASTICITY    OP   THE   AIR. 

13.  This  property  of  the  air  has  already  been  explained  in  Hydro- 
statics, Art.  4,  and  also  in  Exp.  1,  Art.  4,  of  the  present  treatise.  The 
following  simple  experiments  will  still  further  elucidate  the  subject. 

EXPERIMENTS. 

Exp.  1.  Introduce  water  into  a  large,  wide-mouthed 
bottle ;  fit  a  small  glass  tube,  open  at  both  ends,  to  the 
mouth  of  this  bottle,  by  means  of  a  perforated  cork,  as 
shown  in  the  figure ;  WOAV  through  the  tube  so  as  to  in- 
crease the  quantity  of  air  in  the  bottle  :  after  withdrawing 
the  mouth  the  water  will  rise  in  a  jet,  owing  to  the  expan- 
sive force  of  the  condensed  air  in  the  bottle. 

Exp.  2.  Fig.  19,  A  is  a  two-necked  bottle  containing 
some  water ;  B  is  an  inflated  bladder  tied  to  one  of  the 
mouths  of  the  bottle ;  a  b  is  a  long  glass  tube  reaching  nearly 


Fig.  18. 


106 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


to  the  bottom  of  the  bottle ;  this  tube  is  fitted  air-tight 
to  the  mouth  of  the  bottle  by  passing  through  a  per- 
forated cork.  By  compressing  the  air  in  the  bladder 
the  water  will  rise  up  the  tube,  from  the  elasticity  or 
pressure  of  the  condensed  air  in  the  bottle. 

Exp.  3.  The  following  instructive  experiment  affords 
an  amusing  illustration  of  the  elasticity  of  the  air,  as 
well  as  of  the  nature  of  specific  gravity.  Fig.  20,  A  is 
a  wide-mouthed  bottle,  nearly  filled  with  water,  in 
which  some  hollow  glass  figures  having  a  hole  in  one 
foot,  called  bottle  imps,  are  placed  so  as  to  float  near 
the  surface  when  filled  with  air  ;  a  piece  of  bladder  is 
tied  over  the  mouth  of  the  bottle  so  as  to  exclude  the 
external  air.  Press  the  bladder  with  the  fingers ;  the 
figures  descend  in  the  water ;  remove  the  pressure,  and 
they  ascend  ;  and  so  on.  By  thus  alternately  raising 
and  depressing  the  fingers,  the  little  figures  may  be 
made,  as  it  were,  to  dance  up  and  down  the  fluid. 
Here  the  pressure  on  the  bladder,  by  compressing  the 
air  beneath  it,  produces  a  pressure  on  the  surface  of 
the  water,  and  this  causes  a  small  portion  of  the  liquid 
to  enter  the  hollow  figures,  which  increases  their  spe- 
cific gravity,  and  in  this  case,  therefore,  they  descend ; 
on  the  contrary,  when  the  pressure  is  removed  from 
the  bladder,  the  air  within  the  figures  regains  its  ori- 
ginal bulk,  and  then  they  ascend.  On  this  principle 
fishes  are  enabled  to  rise  and  fall  in  the  water :  they 
have  a  little  air  bladder  within  their  bodies,  which 
they  contract  wrhen  they  wish  to  descend,  and  expand  when  they  wish 
to  rise. 

Exp.  4.   Invert  a  small  bottle,  and  introduce  so  much 
water  as  will  just  cause  it  to  float  on  the  surface  of  the  fluid » 
gently  depress  the  bottle  to  about  the  middle  of  the  water, 
without  allowing  any  of  the  air  to  escape  :  the  bottle  sinks 
to  the  bottom,  where  it  will  remain.     In  fact,  the  bottle  will 
only  float  near  the  surface.     Here,  when  the  bottle  is  de- 
pressed, an  additional  portion  of  water  enters  it,  in  consequence  of  the 
increased  depth  of  the  fluid  ;  by  this  means  the  specific  gravity  of  the 
bottle  is  increased,  and  hence  it  sinks. 

Exp.  5.    The  popgun  affords  a  good  illustration  of  the  elasticity  of 
the  air. 

14.   The  .elasticity  or  pressure  of  air  increases  with  the 
decrease  of  the  space  which  it  is  forced  to  occupy. 


Fig.  20. 


Fig.  21. 


PNEUMATICS. 


107 


Fig.  22. 


In  order  to  explain  this  law,  let  P  represent  a  piston 
compressing  the  air  in  the  cylinder  A  B  C  D  ;  sup- 
pose the  surface  of  the  piston  to  be  one  square  inch,  and 
that  the  atmosphere  exerts  a  pressure  of  15  Ibs.  per 
square  inch ;  now  let  an  additional  load  or  pressure  of 
15  Ibs.  be  laid  on  the  piston,  then  the  piston  will  de- 
scend to  a  6,  and  the  air  beneath  it  will  be  reduced  to 
one  half  its  original  volume  —  that  is  to  say,  air  under 
a  pressure  of  two  atmospheres  is  reduced  to  yne  half  its  , 
original  volume.  Again,  let  twice  15  Ibs.  be  laid  upon 
the  piston,  then  it  will  descend  still  farther,  and  the  air  cf 
beneath  it  will  be  reduced  to  one  third  its  original 
space  —  that  is  to  say,  air  under  a  pressure  of  three  at- 
mospheres is  reduced  to  one  third  its  original  space ;  and  so  on.  Thus  it 
appears  that,  as  we  increase  the  pressure  applied,  so  we  in  the  same  pro- 
portion reduce  the  space  occupied  by  the  air.  And  it  will  be  readily 
understood  that  the  pressure  which  compresses  any  portion  of  air  is  the 
measure  of  its  elasticity  or  tendency  which  it  has  to  expand. 

This  law  of  elasticity  was  first  proved  by  Marriotte,  in  the  following 
manner :  — 

Experiment.  Take  a  bent  tube  H  E  A  B  closed  at  B ; 
introduce  a  little  mercury,  so  as  to  make  it  stand  at  the 
same  level  E  A  in  both  legs  of  the  tube  ;  let  the  space  A  B 
occupied  by  the  enclosed  air  be  divided  into  equal  parts ; 
pour  mercury  into  the  tube  until  the  volume  of  air  in  A  B 
is  reduced  to  C  B  ;  then  it  will  be  found  that  when  C  B  is 
one  half  A  B,  the  column  of  mercury  D  H  producing  this 
compression  is  about  30  inches,  or  a  column  of  mercury 
which  balances  the  pressure  of  the  atmosphere ;  that  when 
C  B  is  one  third  A  B,  or  when  the  volume  of  air  is  reduced 
three  times,  the  column  of  mercury  D  H  is  twice  30  inches, 
and  so  on ;  thereby  proving  the  law  of  elasticity  just  ex- 
plained. 


Variation  in  the  Density  of  the  Air. 


Fig.  23. 


15.  It  has  been  already  mentioned,  Art.  10,  that  as  we  rise  above  the 
earth's  surface  the  air  becomes  thinner  and  thinner,  or  less  and  less  dense 
this  is  a  necessary  consequence  of  the  law  of  elasticity.  The  follow- 
ing remarkable  relation  between  the  density  of  the  air  and  its  height 
above  the  level  of  the  sea  deserves  to  be  especially  noticed :  as  the  eleva- 
tion above  the  level  of  the  sea  increases  in  arithmetical  progression,  the 
density  or  pressure  of  the  air  decreases  in  geometrical  progression.  Thus, 
if  the  pressure  of  the  air  at  the  level  of  the  sea  be,  on  an  average,  15  Ibs. 


108          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

per  square  inch,  then  at  the  height  of  about  3£  miles  *  it  has  a  pressure  of 
£  of  15  Ibs. ;  at  the  height  of  2  times  3£  miles  it  has  a.  pressure  of  £  of  15 
Ibs. ;  at  the  height  of  3  times  3£  miles  it  has  a  pressure  of  £  of  15  Ibs. ; 
and  so  on.  It  will  be  seen  that  for  every  successive  3£  miles  which  we 
ascend,  the  pressure  of  the  air  is  always  the  half  of  what  it  is  at  the  pre- 
ceding elevation.  This  law  would  be  strictly  true,  if  the  atmosphere 
were  every  where  of  the  same  temperature  and  contained  the  same  quan- 
tity of  moisture. 

Relations  of  Air  to  Heat. 

16.  When  a  body  is  heated,  it  expands  or  becomes  greater 
in  bulk ;  in  this  way,  heat  rarefies  bodies,  and  causes  them  to 
become  specifically  lighter. 

Elastic  fluids,  such  as  air,  are  more  susceptible  of  this  action  of  heat 
than  either  solids  or  liquids.  The  air  over  a  common  fire  becomes  rare- 
fied by  the  heat,  and  being  thus  rendered  specifically  lighter  than  the 
surrounding  atmosphere,  it  ascends  up  the  chimney,  and  its  place  is 
supplied  by  the  current  of  air  which  rushes  towards  the  fireplace  from 
all  parts  of  the  room,  especially  from  the  openings,  or  apertures  in  win- 
dows and  doors.  Thus  a  fire  creates  an  artificial  wind.  On  the  same 
principle,  the  unequal  distribution  of  heat  over  the  earth  produces  on  a 
great  scale  the  various  currents  of  air  or  winds,  which  are  every  where  felt. 

17.  The  following  simple  experiments  will  render  this  property  of  air 
more  apparent.  'ft 

EXPERIMENTS. 

Exp.  1.  Partially  fill  a  bladder  with  air,  and  after  tying  its  mouth, 
place  it  near  a  good  fire :  the  air  within  the  bladder  expands  and  com- 
pletely fills  it. 

Exp.  2.  Invert  a  wine  glass  in  a  basin ;  gently 
pour  hot  water  into  it :  bubbles  of  air  escape  from 
the  wine  glass,  in  consequence  of  the  expansion 
of  the  air  by  the  heat. 

Exp.  3.  Throw  a  piece  of  burning  paper  into  a 
wine  glass,  and  while  the  paper  is  still  burning, 
forcibly  close  the  mouth  of  the  glass  with  the 
hand ;  after  a  few  seconds,  the  glass  will  be  found  Fig.  24. 

to  stick  to  the  hand  with  considerable  force. 

Here  the  heat  expels  nearly  the  whole  of  the  air  in  the  glass,  by  causing 
it  to  expand  •  after  the  air  in  the  glass  cools,  it  contracts,  and  then  the 

*  More  exactly,  3.42  miles. 


PNEUMATICS. 


109 


Fig.  25. 


pressure  of  the  external  air  upon  the  outside  of  the  glass 
becomes  greater  than  the  pressure  of  the  rarefied  air  within, 
the  glass. 

Exp.  4.  Cut  a  piece  of  paper  in  the  form  of  a  spiral, 
as  in  Fig.  26;  run  a  thread  through  the  centre  c;  sus- 
pend the  paper  by  this  thread,  and  it  will  have  some- 
thing like  the  form  of  a  corkscrew ;  bring  it  over  the 
name  of  a  candle  :  the  suspended  paper  turns  round  in  one 
certain  direction.  Here  the  heated  air  about  the  candle 
ascends,  and  by  striking  against  the  surface  of  the  paper, 
causes  it  to  revolve  on  the  same  principle  as  a  toy  wind- 
mill. 

Ttie  Exhausting  and  Condensing  Syringe. 

18.  This  instrument  is  used  for  two  pur- 
poses, viz.,  for  exhausting  air  from  a  vessel, 
and  also  for  compressing  air  into  a  vessel.  A 
section  of  this  instrument  is  represented  in 
the  accompanying  figure.  P  is  a  solid  piston, 
working  air  tight  in  a  cylinder  :  P  S  is  the 
piston  rod,  working  through  an  air  tight  collar 
S,  so  that  as  the  piston  rod  moves  up  and 
down  through  this  collar,  no  air  shall  be  al- 
lowed to  pass  through  it  into  the  cylinder  ;  V 
is  a  valve,  or  little  door,  opening  outwards  ; 
O  is  an  open  aperture  leading  to  the  vessel 
A,  from  which  air  is  to  be  exhausted.  Let  us  now  see  how  this  in- 
strument exhausts  the  air  from  vessels.  First  of  all,  the  piston  P  is 
drawn  to  the  top  of  the  cylinder,  then  the  glass  globe  A,  having  a  stop 
cock  B  attached  to  it,  is  screwed  on  to  the  pipe  O,  and  the  stop  cock  B 
is  opened.  The  instrument  being  in  this  state,  force  down  the  piston ; 
then  the  air  beneath  it  is  driven  out  of  the  cylinder,  through  the  valve 
V,  while  the  air  in  the  globe  expands  and  fills  the  upper  part  of  the  cyl- 
inder. Raise  the  piston  ;  then  the  valve  V  is  closed  by  the  pressure  of 
the  external  air,  and  a  vacuum  is  formed  beneath  it ;  but  the  moment 
the  piston  P  passes  the  orifice  O,  the  air  rushes  from  the  bottle  and  fills 
up  the  void  formed  in  the  cylinder.  When  the  piston  is  forced  down 
again,  a  quantity  of  air,  equal  to  the  volume  of  the  cylinder,  is  again 
driven  out ;  so  that  after  this  operation  has  been  repeated  for  about  a 
dozen  times,  the  air  in  the  bottle  becomes  so  attenuated  or  rarefied,  as 
almost  to  approach  a  vacuum.  After  the  exhaustion  is  completed,  the 
cock  B  is  closed,  and  the  globe  is  unscrewed  from  the  cylinder. 
10 


Fig.  27. 


110 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Let  u:-  now  see  how  the  instrument  acts  as  a  condenser  of  air.  First 
of  all,  the  piston  P  is  drawn  to  the  top  of  the  cylinder ;  then  the  bottle 
A,  into  which  the  air  is  to  be  compressed,  is  screwed  on  to  the  pipe  Q, 
the  pipe  O,  in  this  case,  being  left  completely  open.  Force  down  the 
piston ;  then  the  air  beneath  it  is  driven  through  the  valve  V  into  the 
bottle.  Raise  the  piston  ;  then  a  vacuum  is  formed  beneath  it,  but  at 
the  same  time  the  valve  V  is  kept  shut  by  the  pressure  of  the  air  in  the 
bottle,  so  that  no  air  can  escape  from  it ;  now  the  moment  the  piston  P 
passes  the  orifice  O,  the  external  air  rushes  into  the  cylinder  and  fills  it. 
In  the  next  downward  stroke,  the  air  beneath  the  piston  is  again  forced 
into  the  bottle ;  so  that  at  every  downward  stroke  a  quantity  of  air, 
equal  in  volume  to  the  cylinder,  is  forced  into  the  bottle.  >V hen  the 
air  has  been  sufficiently  condensed,  the  cock  B  is  closed,  and  the  bottle 
is  unscrewed  from  the  cylinder.  The  bottles  used  for  holding  condensed 
air  are  usually  made  of  metal. 

The  Air  Pump. 

19.  The  air  pump  is  used  for  withdrawing  the  air  from  large  glass 
vessels,  called  receivers,  in  which  experiments 
are  performed.  The  accompanying  figure 
represents  a  common  air  pump,  with  a  single 
ban-el.  P  is  a  piston,  working  air  tight  in 
the  barrel  or  cylinder  II  e ;  this  piston  has 
a  valve,  or  little  door  in  it,  opening  up- 
wards, which  allows  the  air  to  escape  out- 
wards, but  does  not  allow  any  air  to  pass 
inwards  ;  V  is  a  valve,  placed  at  the  bottom 
of  the  cylinder,  which  also  lifts  upwards; 
e  D  E  O  is  a  pipe,  which  connects  the  cylin- 
der with  a  flat,  polished  plate  B,  on  which 
the  receiver  A  stands ;  the  bottom  of  this 
receiver  is  ground  flat,  so  that  it  may  fit  per- 
fectly air  tight  to  the  plate  when  a  little  lard 
is  rubbed  over  it ;  K  is  a  stop  cock ;  e  is  a 
nut,  which,  being  unscrewed,  allows  the  ex- 
ternal air  to  enter  the  receiver  ;  N  M  is  the 


Fig.  28. 


mercury  gauge  for  indicating  the  degree  of  exhaustion  produced  in  the 
receiver  A ;  this  gauge  acts  on  the  same  principle  as  the  Torricellian 
tube.  (See  Art.  10.) 

Let  us  now  see  how  the  pump  acts.  The  receiver  A,  from  which  the 
air  is  to  be  withdrawn,  being  carefully  placed  upon  the  plate  with  a 
little  clean  lard  rubbed  upon  it,  the  stop  cock  K  is  opened,  and  the  nut 
e  is  screwed  tightly  up.  The  instrument  being  in  this  state,  the  piston 


PNEUMATICS. 


Ill 


P  is  worked  rapidly  up  and  down,  until  a  sufficient  degree  of  exhaus- 
tion is  produced  in  the  receiver,  which  is  always  shown  by  the  height 
N  M  at  which  the  mercury  stands  in  the  gauge ;  the  stop  cock  K  is 
then  closed  in  order  to  cut  off  any  further  communication  with  the 
pump.  At  each  downward  stroke  of  the  piston,  the  valve  in  it  opens, 
allowing  the  air  beneath  it  to  escape,  while  the  valve  V  is  closed ;  on 
the  contrary,  at  each  upward  stroke,  the  valve  in  the  piston  is  closed  by 
the  pressure  of  the  external  air,  while  the  air  in  the  receiver  lifts  up  the 
valve  V,  and  fills  up  the  vacuum  which  would  otherwise  be  formed  be- 
neath the  piston.  Thus  a  certain  portion  of  the  air  remaining  in  the 
receiver  is  always  withdrawn  at  every  double  stroke,  so  that  by  contin- 
uing the  process,  the  air  in  the  receiver  at  length  becomes  so  rarefied  as 
almost  to  approach  a  vacuum. 

To  show  the  use  of  the  gauge,  let  us 
suppose  that  the  column  of  mercury  in 
the  barometer  stands  at  the  height  of  30 
inches,  and  that  the  column  M  N  in  the 
gauge  is  28  inches  ;  then  the  deficiency,  2 
inches,  is  due  to  the  elasticity  of  the  air  in 
the  receiver  ;  and,  theKfefore,  since  2  is  the 
•j-1^  part  of  30,  the  elasticity  of  the  air  in 
the  receiver  will  be  the  -^  part  of  the  elas- 
ticity of  the  external  air. 

20.  In  order  to  facilitate  the  exhaustion, 
air  pumps  are  usually  made  with  two  cyl- 
inders, so  that  while  one  piston  is  ascend- 
ing, the  other  is  descending,  and  thus  the 
process  of  exhaustion  is  continually  kept 
up.  The  pistons,  in  these  pumps,  are 
moved  by  a  toothed  wheel,  which  is  made 
to  act  upon  racks  formed  upon  the  piston 
rods.  The  accompanying  figure  represents 
an  air  pump  of  this  kind,  a  and  e  are  the 
two  barrels ;  r  and  R  the  racks  formed  on 
the  piston  rods  ;  H  is  the  handle  or  winch, 
which  gives  motion  to  the  toothed  wheel 
placed  between  the  racks,  so  that  a  back 
and  forward  motion  being  given  to  this 
handle,  an  up  and  down  motion  is  commu- 
nicated to  the  pistons ;  A  is  the  receiver,  Fig.  29. 
standing  on  the  plate  B  ;  T  is  a  table,  on 

which  the  machine  is  fixed ;  E  E  are  the  pillars  supporting^the  plate  B  ; 
M  is  the  mercury  gauge ;  and  so  on  to  the  other  parts  of  the  machine, 
as  already  described. 


112 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  30. 


EXPERIMENTS  PERFORMED  WITH  THE  AIR  PUMP. 

Exp.  1.  To  fasten  the  hand  to  a  glass  by  means  of 
the  atmospheric  pressure.  A  B  is  a  small  glass,  open 
at  both  ends,  about  3  inches  diameter.  Place  this  glass 
over  the  hole  of  the  air  pump  plate;  lay  your  hand 
tightly  over  the  top  A ;  turn  the  handle  of  the  pump 
for  a  few  times:  the  hand  becomes  fastened  to  the 
glass  by  the  pressure  of  the  air. 

Exp.  2.   To  burst  a  bladder  by  the  pressure  of  the 
atmosphere.     Tie  a  piece  of  thin  bladder,  moistened  in 
water,  over  one  end  of  the  glass  used  in  the  last  experiment ; 
after  the  bladder  has  become  dry,  it  will  be  perfectly  tight. 
Place  the  glass  on  the  plate  of  the  air  pump ;  then,  after  a 
few  turns  of  the  handle,  the  bladder  will  burst  with  a  loud 
report,  from  the  pressure  of  the  atmosphere. 

Exp.  3.  A  and  B  are  two  brass  cups,  called  Magdebourg 
hemispheres,  which  exactly  fit  each  other  at  the  edges,  so 
that  when  they  are  brought  together  they  form  a  sphere ;  C 
is  a  pipe,  with  a  stop  cock  leading  into  the  in%rior  of  the 
cup  B.  Put  a  little  lard  on  the  edges  of  the  cups,  and  bring 
them  together  ;  screw  them  by  means  of  C  to  the  plate  of 
the  air  pump ;  exhaust  the  air  from  the  inside ;  turn  the 
stop  cock  C,  and  unscrew  them  from  the  pump ;  screw  the 
handle  D  on  at  C  :  the  cups  being  now  pressed  together  by 
the  atmosphere,  will  require  a  considerable  force  to  separate 
them. 

Supposing  the  air  to%e  completely  exhausted  from  the 
inside  of  the  cups,  and  that  their  section  contains  10  square      -"Q 
inches;  then  the  atmospheric  pressure  on  each  square  inch         (Q) 
will  be  about  15  Ibs.,  and  therefore  the  whole  pressure  of  the      Fig.  32. 
atmosphere,  tending  to  keep  the  cups  together,  will  be  10 
times   15  Ibs.,  or   150  Ibs.     In  this  case,  therefore,  it  would  require  a 
weight  of  150  Ibs.  to  separate  the  cups. 

Exp.  4.  Tie  the  mouth  of  a  little  flaccid  bladder  ;  place  it  beneath 
the  receiver  of  an  air  pump ;  exhaust  the  air  from  the  receiver :  the  air 
within  the  bladder  gradually  expands  (the  pressure  of  the,  air  within 
the  receiver  being  removed)  until  the  bladder  becomes  completely  dl?- 
tendcd  ;  allow  the  external  air  to  enter  the  receiver  by  turning  the 
screw  K,  (see  Fig.  28  :)  the  bladder  becomes  shrivelled  up  as  at  first. 

Exp.  5.  Put  a  glass  bnlb  B,  blown  at  the  end  of  a  tube,  into  a  bottle 
of  water,  as  shown  in  the  figure ;  place  them  beneath  the  receiver  of 
the  air  pump  ;  exhaust  the  air  from  the  receiver ;  then,  as  the  exhaustion 


PNEUMATICS. 


113 


Fig.  33. 


goes  on,  the  air  in  the  bulb  will  rise  in  bubbles  through  the 
water,  so  that  the  air  in  the  bulb  will  become  rarefied,  as  well 
as  that  which  is  in  the  receiver.  When  the  bubbling  has 
ceased,  allow  the  external  air  to  enter  the  receiver  :  the  water, 
from  the  atmospheric  pressure,  rushes  into  the  bulb,  and 
nearly  fills  it. 

Exp.  6.  A  small  bottle  containing  a  bubble  of  air  is  sunk  in 
a  deep  vessel  filled  with  water,  as  in  the  accompanying  figure ; 
place  the  vessel  beneath  the  receiver  of  the  air  pump,  and  ex- 
haust the  air :  the  bottle  rises  in  the  water ;  allow  the  air  to 
enter  the  receiver :  the  bottle  sinks  to  the  bottom;  and  so  on. 
For  an  explanation  of  this  experiment,  see  Exp.  3,  Art.  13. 

Exp.  *l.   A  represents  a  receiver,  open  at  the  top,  but  which 
is  closed  air  tight  by  the  perforated  cork  &,  and  barometer  tube     l$' 
ab ;  c  is  a  cup  of  mercury,  into  which  the  open  extremity 
of  the  tube  a  b  nearly  dips.     Exhaust  the  air  from  the 
receiver  ;  depress  the  tube  a  b,  so  that  its  extremity  may  be 
immersed  in  the  mercury ;  allow  the  external  air  to  enter 
the  receiver :  the  mercury  mounts  up  the  tube  a  b  very 
nearly  to  the  height  of  30  inches. 

This  experiment  clearly  shows,  that  mercury  is  sustained 
in  the  barometer  tube  by  the  pressure  of  the  atmosphere 
alone,  and  not  by  any  imaginary  principle,  such  as  suction 
or  nature  s  horror  of  a  vacuum,  as  the  ancient  philosophers 
supposed.  (See  Art.  10.) 

Exp.  8.  The  accompanying  figure  represents  a  piece  of 
apparatus  for  producing  a  fountain  in  a  vacuum.  A  brass 
pipe  a  e  passes  through  a  smooth  plate  B  ;  this  pipe  has  a 
stop  cock  at  C,  and  a  jet  at  its  upper  extremity  e;  R  is  a 
tall  glass  receiver,  standing  on  the  plate  B,  from  which  the 
air  may  be  withdrawn  by  screwing  the  extremity  a  of  the 
pipe  into  the  hole  of  the  air  pump  plate.  When  the  air 
has  been  withdrawn  from  the  receiver  R,  it  becomes  fixed 
to  the  plate  B  ;  the  stop  cock  C  is  then  closed,  and  the  ap- 
paratus is  unscrewed  from  the  pump.  Now  plunge  the 
extremity  a  of  the  pipe  into  a  vess^  of  water ;  open  the 
cock  C  :  the  water  rises  in  a  beautiful  jet  within  the  receiver. 

Exp.  9.  To  transfer  a  liquid  from  one  bot- 
tie  to  another.  The  bottle  A  contains  some 
colored  liquid;  the  bent  tube  A  a  b  B, 
reaching  nearly  the  bottom  of  the  bottles,  is 
fitted  air  tight  to  the  neck  of  the  bottle  A, 
but  passes  freely  through  the  neck  of  the 
10* 


Fig.  37. 


114 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


other  one.  Place  this  apparatus  under  the  receiver  of  the  air  pump,  and 
exhaust  the  air  :  the  liquid  passes  from  A  to  B,  from  the  elasticity  of 
the  air  in  the  former.  Now  admit  the  air  into  the  receiver  :  the  liquid 
returns  to  the  bottle  A. 

Exp.  10.  Place  a  shrivelled  apple  beneath  the  receiver  of  the  air 
pump ;  exhaust  the  air  :  the  apple  gradually  becomes  plump  and  rounded, 
from  the  expansion  of  the  air  within  it.  Admit  the  air  into  the  receiver : 
the  apple  becomes  shrivelled  up  as  at  first. 

Exp.  11.  Place  a  glass  of  beer  beneath  the  receiver;  exhaust  the  air 
from  it :  the  beer  foams  up  and  appears  quite  brisk,  from  the  escape  of 
carbonic  acid  gas  which  is  in  it.  Now  admit  the  air  into  the  receiver ; 
the  bubbling  ceases,  and  the  beer  appears  flat  and  dead. 

Exp.  12.  To  show  that  air  is  contained  in  the  pores  of  solid  sub- 
stances. Put  a  piece  of  beet  root,  or  any  porous  substance,  into  a  vessel 
of  water,  and  place  it  beneath  the  receiver  of  the  air  pump ;  then,  upon 
exhausting  the  receiver,  the  beet  root  becomes  covered  with  little  globules 
of  air,  which  at  once  disappear  when  the  external  air  is  readmitted  into 
the  receiver. 

Exp.  13.   The  pressure  of  the  atmosphere  will  force  mercury  through 
the  pores  of  wood.    The  metal  plate  a  a  is  made  to  fit  the 
top  of  a  receiver ;  this  plate  has  a  hole  passing  through 
it,  into  which  is  fitted  a  wooden  cup  b.     Place  the  plate  a 
and  cup  upon  the  top  of  the  receiver ;  fill  the  cup  b  with 
mercury,  and  exhaust  the  air  from  the  receiver :  a  fine 
shower  of  mercury  falls  into  the  receiver. 

Exp.  14.   In  highly  rarefied  air,  a  feather  falls  as  quickly  as  a  guinea. 

A  is  a  long  receiver,  placed  upon  the  plate  of  the  air 
pump ;  a  is  a  metal  plate  covering  the  top  of  the  receiver ; 
s  s  are  two  flaps,  suspended  from  the  plate  a,  on  which  the 
feather  and  coin  are  laid;  the  wire  r  passes  through  the 
plate,  and  carries  a  stage,  with  two  notches  in  it,  at  the  lower 
end,  for  supporting  the  flaps.  Having  turned  up  the  flaps, 
place  the  feather  and  coin  upon  them ;  exhaust  the  air  from 
the  receiver ;  turn  the  wire  r  until  the  flaps  slip  down 
through  the  notches  in  the  stage ;  the  feather  and  the  coin 
drop  at  the  same  instant,  and,  faUpg  with  equal  velocities, 
they  reach  the  bottom  of  the  receiver  in  the  same  time. 

Exp.  15.   Air.  resists  the  motion  of  machinery.     Here  a 
and  b  are  two  wheels  of  the  same  size,  turning  on  separate     pig.  39. 
axes ;  but  the  vanes  of  a  cut  the  air  edgewise,  while  the 
vanes  of  b  strike  it  breadthwise ;  by  suddenly  raising  or  depressing  the 
rod  d  e,  a  rapid  rotatory  motion  is  given  to  the  two  wheels ;  this  rod 
passes  through   an  air  tight  stuffing  box  e,  placed  at  the  top  of  the 


Fig.  38. 


PNEUMATICS. 


115 


Fig.  40. 


receiver  R.  Let  motion  be  given  to  the  wheels 
when  the  receiver  contains  air  :  the  wheel  b  stops 
much  sooner  than  a.  Now  exhaust  the  receiver, 
and  then  set  the  wheels  in  motion ;  the  wheels 
continue  to  move  for  a  much  longer  time  than 
they  did  in  the  air  ;  and  moreover  they  stop  at 
the  same  instant. 

Exp.  16.  Smoke  falls  in  rarefied  air.  Blow 
out  a  candle,  and  put  it  under  a  receiver ;  the 
smoke  rises  to  the  top.  Partially  exhaust  the 
air  from  the  receiver ;  the  smoke  descends  in 
the  fluid  specifically  lighter  than  itself. 

Exp.  17.  Weighed  in  the  air,  an  ounce  of 
cork  is  heavier  than  an  ounce  of  lead.  Balance  a  piece  of  cork  and 
lead  in  a  small  pair  of  scales ;  place  them  beneath  the  receiver,  and 
exhaust  the  air ;  the  scale  on  which  the  cork  is  put  plainly  preponder- 
ates. This  shows  that  the  air  exerts  a  greater  force  of  buoyancy  on  the 
cork  than  it  does  on  the  lead. 

Exp.  18.  Sound  is  not  transmitted  through  highly  rarefied  air.  To 
show  this  important  fact,  a  bell  must  be  placed 
upon  some  bad  conductor  of  sound,  such  as 
wool  or  horse  hair,  to  separate  it  from  the  plate 
of  the  air  pump ;  and  the  apparatus  must  be  so 
contrived  that  the  clapper  can  be  made  to  strike 
the  bell  without  allowing  the  external  air  to 
enter  the  exhausted  receiver.  In  the  accompa- 
nying figure,  R  represents  the  receiver ;  a  the 
bell,  standing  on  the  horse  hair  cushion  g ;  c  b 
the  clapper,  which  may  be  agitated  by  the  lever 
h,  attached  to  the  rod  h  k,  passing  through  the 
stuffing  box  s  at  the  top  of  the  receiver.  Before 
the  air  is  withdrawn  from  the  receiver,  let  the 
clapper  be  agitated,  to  show  that  the  sound  of 


Fig.  41. 


the  bell  is  distinctly  transmitted  through  the  air  in  the  receiver.  Now 
let  the  air  be  exhausted,  and,  during  the  process,  let  the  clapper  be  agi- 
tated from  time  to  time ;  the  sound  becomes  more  and  more  feeble,  until 
it  ceases  altogether. 

Exp.  19.  Water  boils  at  a  much  lower  temperature  in  rarefied  air. 
Place  some  hot  water  beneath  the  receiver,  and  exhaust  the  air :  the 
water  boils  violently.  Admit  the  air  into  the  receiver ;  the  ebullition  in 
a.  moment  ceases ;  and  so  on. 

Hence  it  is  that  water  boils  at  a  much  lower  temperature  at  the  top 
of  a  mountain  than  it  does  at  the  level  of  the  sea.  ^ - 

/  ^\ 

(i    UNIVERSITY    I 

V  y 


116         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  42. 


PNEUMATIC    AND    HYDRAULIC    MACHINES. 

The    Common  Pump. 

21.  The  accompanying  figure  represents  a  section  of  the  common 
suction  purnp.  A  C  is  a  cylinder  or  barrel,  in 
which  a  piston  P  is  moved  up  and  down  by 
means  of  a  piston  rod  R,  attached  to  the  ex- 
tremity of  the  lever  E,  H  of  the  first  kind.  In 
the  piston  is  a  valve  v  lifting  upwards  ;  and  at 
the  bottom  of  the  barrel  is  another  valve  V,  also 
lifting  upwards.  A  B  is  a  pipe,  passing  from 
the  bottom  of  the  barrel  into  the  well  from  which 
the  water  is  to  be  raised. 

The  first  effect  of  the  motion  of  the  piston  is 
to  clear  the  barrel  and  pipe  of  air ;  at  the  first 
upward  stroke  of  the  piston,  the  air  in  the  pipe 
A  B  expands  and  enters  the  barrel,  and  being 
thus  rarefied,  exerts  less  pressure  upon  the  water 
in  the  pipe ;  the  consequence  is,  that  the  pressure 
of  the  external  air  forces  a  portion  of  water  into 

the  pipe.  Now,  in  the  downward  stroke  of  the  piston,  the  valve  V 
closes,  while  v  opens  and  allows  the  air  in  the  barrel  to  escape,  so  that 
there  is  now  a  much  less  quantity  of  air  in  the  pipe  than  there  was  at 
first;  at  the  second  upward  stroke,  therefore,  the  air  in  the  pipe  is  still 
further  rarefied,  and  thus  an  additional  quantity  of  water  is  raised  in 
the  pipe  by  the  pressure  of  the  external  air  ;  proceeding  in  this  manner, 
after  a  few  strokes,  the  water  is  raised  into  the  barrel,  and  then  another 
kind  of  action  takes  place. 

In  a  downward  stroke  of  the  piston,  it  plunges  amongst  the  water  in 
the  barrel  of  the  pump ;  the  valve  V  closes,  and  the  valve  v  opens,  and 
allows  the  water  to  pass  to  the  upper  side  of  the  piston.  In  an  upward 
stroke,  the  valve  v  closes,  and  the  valve  V  opens,  and,  by  the  pressure 
of  the  atmosphere,  the  water  follows  the  piston  in  its  ascent,  whereas 
the  water  above  the  piston  is  pushed  before  it,  and  thus  the  fluid  is  dis- 
charged in  a  stream  at  the  mouth  C  of  the  pump ;  and  so  on  to  any 
number  of  strokes. 

If  a  perfect  vacuum  were  formed  by  the  piston  as  it  ascends,  the  water 
would  be  raised,  on  an  average,  to  the  height  of  34  feet  above  the  level 
of  the  water  in  the  well,  which  is  the  height  of  a  column  of  water  cal- 
culated to  balance  the  average  pressure  of  the  atmosphere. 


PNEUMATICS. 


117 


The   Common  Forcing  Pump. 

22.   This  pump  raises  water  from  the  well  into  the  barrel,  on  the  prin- 
ciple of  the  suction  pump  just  described,  and  then 
the  pressure  of  the  piston  on  the  water  elevates  it 
to  any  height  that  may  be  required. 

Here  P  is  a  solid  piston,  working  up  and  down 
in  a  barrel  ;  V  a  valve,  lifting  upwards,  placed  at 
the  top  of  the  pipe  descending  into  the  well  ;  v  a 
valve,  also  lifting  upwards,  placed  in  a  pipe  D, 
which  conveys  the  water  to  the  cistern. 

In  a  descending  stroke  of  the  piston,  the  valve  V 
closes,  and  the  valve  v  opens,  and  the  water,  being 
pressed  before  the  piston,  is  forced  up  the  pipe  D  to 
the  higher  level  required  ;  on  the  contrary,  in  an 
ascending  stroke,  the  valve  v  closes  by  the  pressure  of  the  external  air 
and  the  water  in  the  pipe  D  ;  the  valve  V  opens,  and  the  water  rises  into 
the  barrel  of  the  pump  by  the  pressure  of  the  atmosphere  on  the  water 
in  the  well  ;  and  so  on  to  any  number  of  strokes. 


Fig.  43. 


an 


Air   Chambe 


The  Forcing  Pump  with 

23.  This  engine  merely  differs  from  the  preceding  one  by  having  an 
air  chamber  e  c  v  connected  with  the  vertical  pipe  D. 
This  air  chamber  is  a  closed  vessel,  having  the  pipe 
D  descending  into  it,  and  a  valve  v  opening  and 
closing  its  communication  with  the  barrel  of  the 
pump.  When  the  piston  P  descends,  the  water  is 
forced  through  the  valve  v  into  the  air  chamber,  so 
that  as  soon  as  the  water  rises  above  the  lower  ori- 
fice of  the  pipe  D,  the  air  in  the  upper  part  of  the 
chamber  is  contracted  or  compressed  ;  and  this  com- 
pression of  the  air  causes  it  to  exert  a  continuous 
pressure  upon  the  surface  of  the  water  in  the  cham- 


Fig.  44. 


ber,  which  forces  the  fluid  up  the  pipe  D,  and  thus  a  constant  discharge 
into  the  cistern  is  sustained.  In  the  common  forcing  pump,  the  water 
is  only  discharged  at  each  downward  stroke  of  the  piston,  whereas,  in 
the  present  case,  the  pressure  of  the  air  in  the  chamber  sustains  the  dis- 
charge through  the  vertical  pipe  D,  during  the  intervals  taken  up  by 
the  upward  strokes  of  the  piston. 

The  great  defect  of  this  engine  is  as  follows  :  after  the  pump  has  been 
some  time  in  action,  the  air  in  the  chamber  becomes  absorbed  by  the 
water  passing  through  it,  so  that  at  length  it  is  found  that  nearly  all  the 
air  at  first  in  the  chamber  has  passed  away  with  the  water  discharged 
by  the  pump. 


118 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Double-acting  Pump. 

24.  This  pump  is  designed  to  remedy  the  defect  of  the  preceding  one. 
It  is  simply  a  double-acting  forcing  pump,  similar  in  its  construction  to 
that  described  in  Art.  22.  P  is  a  solid  piston,  which 
moves  up  and  down  in  a  cylinder ;  the  rod  of  this 
piston  passes  through  a  stuffing  box  at  S  for  the  pur- 
pose of  keeping  the  cylinder  air  tight.  On  the  op- 
posite sides  of  the  cylinder  are  two  pipes,  A  B  and 
C  D  ;  where  A  B  descends  into  the  well,  and  C  D 
conveys  the  water  to  the  reservoir.  There  are  four 
valves,  a,  b,  c,  c,  opening  and  closing,  as  the  case 
may  be,  the  communication  of  these  pipes  with  the 
cylinder.  These  valves  all  lift  in  the  same  direction, 
that  is,  to  the  right.  Suppose  the  cylinder  and 
pipes  filled  with  water ;  then,  in  an  upward  stroke 
of  the  piston,  the  valves  a  and  c  are  opened,  and  c 
and  b  are  closed ;  the  water  is  forced  by  the  piston 
through  the  valve  e,  and  then  up  the  vertical  pipe 
CD;  at  the  same  time  the  water,  by  the  atmospheric 
pressure,  rises  up  the  pipe  A,  and  opening  the  valve 


Fig.  45. 


a,  follows  the  piston  in  its  ascent :  on  the  contrary,  when  the  piston 
descends,  the  valves  a  and  e  are  closed,  and  c  and  b  are  opened ;  the 
water  is  then  forced  through  the  valve  c,  up  the  vertical  pipe  C  D,  and 
the  water  from  the  well  enters  the  cylinder  through  the  valve  b,  and  fol- 
lows the  piston  in  its  descent ;  and  so  on  to  any  number  of  strokes. 


The  Fire  JZngine. 


25.  This  engine  is  simply  a  combina- 
tion of  two  forcing  pumps,  having  a 
common  air  chamber,  H,  and  the  same 
suction  pipe  F  descending  to  the  water 
intended  to  supply  the  engine.  (See 
Art.  22.)  The  beam  A  B,  turning  on 
its  centre  of  motion  K,  works  the  two 
pistons  C  and  D  ;  so  that  while  the  one 
is  descending,  the  other  is  ascending, 
thereby  keeping  up  a  continuous  flow  of 
water  into  the  air  chamber  II.  A  flex- 
ible tube  e  L  of  leather,  called  a  hose,  is 
attached  to  the  discharge  pipe,  to  enable 
the  engineer  to  direct  the  stream  of  water 


46. 


PNEUMATICS. 


119 


upon  any  particular  spot.  The  degree  of  compression  attained  by  the 
air  in  the  chamber  regulates  the  velocity  with  which  the  water  is  pro- 
jected from  the  nozzle  L  of  the  hose. 

If,  for  example,  the  air  be  compressed  to  one  half  its  original  bulk, 
then  it  will  act  upon  the  surface  of  the  water  in  the  chamber  with  a 
pressure  equivalent  to  that  of  the  atmosphere,  and  the  water  would  be 
raised  in  the  pipe  e  to  the  height  of  about  34  feet,  or  it  would  be  pro- 
jected from  the  nozzle  L  with  a  velocity  equal  to  that  which  a  body 
would  acquire  in  falling  freely,  by  the  force  of  gravity,  from  this  height. 
(See  Art.  41.) 

The  Hydrostatic  Press. 

28.  The  principle  on  which  the  power  of  this  engine  depends  has 
been  explained  in  the  treatise  on  Hydrostatics, 
Art.  11;  it  only  remains,  therefore,  for  us  to 
notice  some  contrivances  connected  with  its 
operation.  H  C  is  a  lever  of  the  second  kind, 
turning  on  the  fixed  centre  C,  which  works 
the  piston  p  of  the  small  cylinder ;  P  is  the 
large  piston,  which,  by  ascending,  compresses 
the  material  S  placed  between  the  press 
boards ;  A  is  a  pipe  proceeding  from  the  bot- 
tom of  the  small  cylinder  to  a  cistern  of  water ; 
V  is  a  valve  lifting  upwards,  placed  at  the  top 
of  this  pipe ;  v  is  a  valve,  opening  to  the  left, 
placed  in  t"he  pipe  which  connects  the  two 
cylinders. 

In  a  descending  stroke  of  the  piston  p,  the  valve  V  closes,  and  the 
valve  v  opens,  and  the  water  is  forced  into  the  large  cylinder,  which 
causes  the  piston  P  to  ascend  and  compress  the  material  S ;  on  the  other 
hand,  in  an  ascending  stroke  of  the  piston  p,  the  valve  v  closes  by  the 
pressure  of  the  water  in  the  large  cylinder ;  the  valve  V  opens  and 
allows  a  fresh  supply  of  water  to  enter  the  small  cylinder ;  and  so  on,  as 
in  the  common  forcing  pump  described  in  Art.  22. 

Hydraulic  Ram. 

27.  This  elegant  and  useful  contrivance  for  raising  water  may  be  em- 
ployed with  advantage  where  there  is  an  abundant  supply  of  water  with 
only  a  small  descent. 

The  action  of  this  engine  depends  upon  the  great  force  which  is  pro- 
duced whenever  a  body  in  motion  suddenly  meets  with  an  obstacle. 
A  body  of  water  acquires  motion  in  its  descent  throtfgh  an  inclined 


120 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


pipe  A ;  and  the  outlet  k, 
upon  being  suddenly  closed, 
allows  the  motion  accumulat- 
ed in  this  body  of  water  to 
expend  itself  in  forcing  some 
of  the  fluid  in  the  pipe  B  into 
an  air  chamber  d,  whence  it  fiff.  48. 

is  raised  by  the  pressure  of 
the  air  in  the  chamber  to  any  proposed  elevation. 

A  is  an  inclined  pipe  conducting  a  stream  of  water  from  a  reservoir ; 
B  a  horizontal  portion  of  this  pipe,  having  a  valve  e  opening  into  the  air 
chamber  d;  a  is  a  heavy  vaive  which  closes,  when  it  is  lifted  upAvards, 
the  outlet  of  the  water  at  k ;  this  valve  is  so  heavy  that  it  descends  in 
the  quiescent  fluid  by  its  own  weight,  thereby  opening  the  outlet  at  k,  at 
the  same  time  it  is  capable  of  being  lifted  up  by  the  impetus  of  the 
water  as  it  rushes  out  of  the  opening  k  with  the  velocity  acquired  in 
descending  the  inclined  pipe  A. 

The  valve  a  being  first  opened,  the  water,  rushing  out  of  the  orifice  k, 
at  length  acquires  a  velocity  sufficient  to  drive  the  valve  a  upwards, 
thereby  closing  the  orifice  k  •  the  current  of  water  through  k,  being  thus 
suddenly  checked,  expends  the  motion  accumulated  in  it  in  forcing  some 
of  the  fluid  through  the  valve  e  into  the  chamber  d.  Now,  when  the 
water  has  become  quiescent,  the  heavy  valve  a  descends  by  its  own  weight 
and  opens  the  orifice  k  ;  the  water  again  rushes  out  of  the  orifice,  and  so 
on  as  already  described. 

Hero's  fountain. 

28.  The  jet  in  this  fountain  is  produced  by 
the  force  of  compressed  air.  a  and  g  are  two 
vessels  united  by  means  of  pipes ;  the  pipe  e  f, 
proceeding  from  the  basin  n  o,  descends  nearly 
to  the  bottom  of  the  lower  vessel  g ;  the  pipe  h  k, 
passing  from  the  top  of  the  vessel  g,  nearly  reach- 
es the  top  of  the  vessel  a ;  the  jet  pipe  d  c  dips 
into  the  water  in  the  vessel «,  and  rises  above  the 
basin  n  o.  Let  us  now  see  how  the  fountain 
acts.  The  jet  cock  b  being  taken  off,  water  is 
poured  through  the  pipe  d  into  the  vessel  a  until 
it  reaches  the  level  I  m ;  the  dish  n  o  is  then 
filled  with  water,  which,  descending  through  the 
pipe  eft  compresses  the  air  in  the  vessels  g  and 
a ;  the  stop  cock  b  being  now  opened,  the  com- 
pressed air  forces  the  water  up  the  pipe  d  c,  and 
thus  the  jet  c  is  produced.  Fig.  49. 


PNEUMATICS. 


121 


The  Siphon  Fountain. 

29.  The  action  of  this  fountain  depends  upon  the 
principle  of  the  siphon.  Its  construction  is  as  follows  : 
A  is  a  glass  receiver  which  fits  closely  to  the  plate  B, 
through  which  two  tubes  n  m  and  s  r  pass ;  the  lower 
extremity  n  of  the  tube  n  m  is  immersed  in  water,  and 
its  upper  extremity  m  rises  within  the  receiver  A  ;  the 
lower  extremity  5  of  the  tube  r  s  descends  below  the  sur- 
face of  the  water  in  the  vessel  n.  To  show  the  action  of 
the  fountain,  invert  the  apparatus,  and  pour  a  little 
water  through  the  tube  s  r  into  the  receiver  ;  close  the 
aperture  s  with  the  finger,  and  place  the  apparatus  in  the 
position  shown  in  the  figure.  Now,  as  the  column  of 
water  in  r  s  is  longer  than  it  is  in  n  m,  on  the  principle 
of  the  siphon,  Art.  11,  the  water  flows  from  s  ;  but  this 
occasions  the  water  to  fall  in  the  receiver  A,  and  hence 
the  air  in  the  receiver  is  rarefied  ;  the  pressure  of  the  ex- 
ternal air  on  the  water  in  the  vessel  n  forces  the  fluid  up 
the  tube  n  m,  and  thus  a  jet  is  formed  within  the  re- 
ceiver. 


DIFFUSION    OF    GASES. 


Fig.  50. 


30.  By  this  property  is  meant  the  tendency  which  airs  or 
gases  have  to  intermix  with  each  other,  without  regard  to 
their  densities. 

EXPERIMENTS. 

Exp.  1.  Take  a  bottle  of  carbonic  acid  gas,  and  invert  a  sim- 
ilar bottle  of  common  air  over  it ;  then  after  a  few  minutes  the 
carbonic  acid  gas  will  be  equally  diffused  through  the  two  ves- 
sels. Here  the  carbonic  acid  gas,  which  is  1£  times  heavier 
than  common  air,  rises  into  the  upper  vessel  in  opposition  to  its 
gravity. 

Exp.  2.   Tie  a  piece  of  bladder  over  one  end  of  a  wide  tube, 
fill  it,  over  water,  with  hydrogen  gas,  and  allow  the  tube  con- 
taining the  gas  to  stand  for  a  few  minutes.     The  water  will  p.     *, 
gradually  rise  in  the  tube,  apparently  in  opposition  to  gravity. 
Here,  from  the  principle  of  diifusiveness,  the  hydrogen,  being  thinner 
than  atmospheric  air,  escapes  from  the  tube  through  the  fine  pores  of 
the  bladder  more  rapidly  than  the  external  air  enters  into  it ;  the  conse- 
quence is,  that  the  pressure  of  the  atmosphere  forces  the  water  up  the 
11 


122          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

tube  to  fill  the  void  which  would  otherwise  be  formed.  In  this  experi- 
ment the  diffusiveness  of  gases  appears  to  act  on  a  similar  principle  to 
that  of  endosmose  and  exosmose,  explained  at  page  90. 

31.  The  principle  of  the  diffusiveness  of  gases  is  of  vast 
importance  in  the  economy  of  nature.     For  example,  atmos- 
pheric  air  is  chiefly  a  mixture  of  two  gases,  oxygen  and 
nitrogen ;  but  they  are  so  completely  diffused  in  the  atmos- 
phere, that  every  where  we  find  them  mixed  in  the  same 
relative  proportions. 

LIQUEFACTION    OF    GASES. 

32.  Some  gaseous  bodies,  when  under  great  pressure  and 
cold,  are  found  to  assume  the  liquid  form  :  for  example,  car- 
bonic acid  gas  becomes  a  liquid  when  subjected  to  the  pressure 
of  about  forty  atmospheres;*  and  chlorine  gas  becomes  a 
liquid  at  a  much  lower  pressure.     But  there  are  some  gases, 
such  as  atmospheric   air,  which   have   hitherto  resisted  all 
attempts  to  liquefy  them ;  such  gases  are  called  permanently 
elastic  fluids. 

ACOUSTICS,  OR  THE  SCIENCE  OF  SOUND. 

33.  The  atmosphere  is  the  usual  medium  through  which 
sound  is  conveyed  to  the  ear. 

(1.)  Sound  is  heard  when  any  sudden  shock  or  impulse 
occurs  in  a  body  having  communication,  through  the  air  or 
otherwise,  with  the  ear. 

Common  instances  of  a  single  impulse  are,  the  blow  from  a  hammer, 
the  clap  of  the  hands,  the  crack  of  a  whip,  a  pistol  shot,  or  any  ex- 
plosion. 

(2.)  Impulses  quickly  repeated  cannot  be  separately  at- 
tended to  by  the  ear ;  and  hence  they  appear  as  one  contin- 
ued sound,  of  which  the  pitch  or  tone  depends  on  the  number 

*  Carbonic  acid  has  even  been  brought  to  the  solid  form. 


PNEUMATICS.  123 

occurring  in  a  given  time  :  all  continued  souna  is  but  a  repe- 
tition of  impulses. 

If  a  wheel  with  teeth  be  made  to  turn,  and  to  strike  a  piece  of  quill 
with  every  tooth,  it  will,  when  moved  slowly,  allow  every  tooth  to  be 
seen  and  every  blow  to  be  separately  heard ;  but  increase  the  velocity, 
and  the  eye  will  lose  sight  of  the  individual  teeth,  and  the  ear,  ceasing 
to  perceive  the  separate  blows,  will  at  last  hear  only  a  smooth,  continu- 
ous sound,  called  a  tone,  of  which  the  character  will  change  with  the 
velocity  of  the  wheel. 

(3.)  When  sonorous  bodies  (such  as  glass,  bell  metal,  the 
string  of  a  violin)  are  struck,  a  tremulous  or  vibratory  motion 
takes  place  in  the  body ;  and  this  vibratory  motion,  being 
impressed  upon  the  air,  is  transmitted  to  the  drum  of  the  ear, 
producing  the  sensation  of  sound.  The  following  simple  ex- 
periments show  that  sonorous  bodies  have  this  property :  — 

EXPERIMENTS. 

Exp.  1.  If  a  bell  be  struck,  its  tremulous  motion  may  be  felt  by  gen- 
tly touching  it  with  the  finger.  When  the  finger  is  pressed  against  the 
bell,  the  sound  is  stopped,  because  the  vibrations  of  the  bell  are  inter- 
rupted. 

Exp.  2.  Attach  a  small  piece  of  cork  by  a  string  to  a  bell ;  strike  the 
bell :  the  cork  vibrates  with  the  bell. 

Exp.  3.  Strike  a  tuning  fork ;  touch  the  surface  of  some  mercury  with 
the  end  of  the  fork :  the  surface  of  the  mercury  exhibits  little  undula- 
tions or  waves. 

Exp.  4.  Sprinkle  some  fine  sand  over  a  square  piece  of  window  glass ; 
hold  it  firmly  by  means  of  a  pair  of  pliers,  and  draw  a  violin  bow  down 
the  edge :  the  sand  is  put  in  motion,  and  finally  settles  itself  in  those 
parts  of  the  glass  which  have  the  least  vibratory  motion.  By  changing 
the  point  by  which  the  plate  is  held,  or  by  varying  the  parts  to  which  the 
violin  bow  is  applied,  the  sand  may  be  made  to  assume  different  beauti- 
ful shapes. 

34.  All  sonorous  bodies  are  elastic ;  and  the  pitch  of  the 
tone  which  they  emit  depends  upon  the  number  of  vibrations 
which  they  perform  in  a  given  time. 

In  all  musical  sounds  the  vibrations  of  the  sonorous  bodies  are  regular, 
that  is  to  say,  the  rapidity  of  their  vibrations  remains  unchanged.  The 


124          NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

frequency  of  vibrations  in  strings  increases  with  their  tension,  shortness, 
and  lightness.  By  tightening  the  string  of  a  violin  the  pitch  of  the  note 
is  raised,  and  the  same  effect  is  produced  by  shortening  the  string ;  in 
both  cases  the  string  is  made  to  vibrate  quicker.  The  pitch  of  the  note 
also  depends  upon  the  thickness  of  the  string ;  for  example,  the  thinnest 
strings  in  the  violin  emit  the  highest  sounds.  In  order  to  produce  a 
musical  sound,  the  number  of  vibrations  performed  by  the  string  cannot 
be  less  than  32  per  second ;  a  string  which  vibrates  twice  as  fast  emits  a 
note  an  octave  higher,  and  a  string  which  vibrates  three  times  as  fast 
emits  a  note  two  octaves  higher,  and  so  on.  When  the  strings  vibrate 
with  the  same  frequency,  the  tones  which  they  emit  are  in  unison.  The 
pleasing  effect  of  harmonious  sounds,  such  as  thirds  and  fifths,  is  pro- 
duced by  the  simplicity  of  the  ratio  of  the  vibrations  performed  in  the 
same  time ;  and,  on  the  contrary,  the  disagreeable  effect  of  discordant 
sounds  arises  from  the  want  of  this  simplicity. 

Where  a  continued  sound  is  produced  by  impulses  which 
do  not,  like  those  of  an  elastic  body,  follow  in  regular  succes- 
sion, the  effect  ceases  to  be  a  clear,  uniform  sound  or  tone, 
and  is  called  a  noise. 

Transmission  of  Sound. 

35.  The  experiment  of  the  bell  in  the  exhausted  receiver 
(see  p.  115)  shows  that  a  sonorous  body  may  vibrate  ;  yet  if 
there  is  no  medium  to  transmit  the  vibrations  to  the  ear,  no 
sound  will  be  produced. 

When  a  gun  is  discharged,  the  sudden  expansion  of  the  powder,  com- 
pressing the  air  immediately  around  it,  produces  a  condensation  of  the 
air  a  little  farther  away ;  this  air,  by  its  elasticity,  expands,  and  in  its 
turn  produces  a  condensation  of  the  air  beyond  it ;  and  so  on  to  a  suc- 
cession of  pulsations  or  waves  created  by  alternate  condensations  and 
expansions  :  in  this  way  all  sounds  are  propagated  through  the  air. 
These  successive  pulsations  or  waves  are  somewhat  lik^  the  successive 
rings  formed  in  water  when  a  stone  is  thrown  into  it. 

36.  Dense  air  is  a  better  conductor  of  sound  than  rare  air. 

Hence  it  is  that  the  sound  of  a  pistol  on  the  top  of  a  high  mountain 
is  scarcely  louder  than  the  crack  of  a  whip. 

The  distance  at  which  a  particular  sound  may  be  heard  depends  upon 
the  state  of  the  atmosphere  with  respect  to  density,  moisture,  &c.  It  is 
on  account  of  the  different  states  of  the  atmosphere  that  St.  Paul's  clock 


PNEUMATICS.  125 

is  heard  so  much  more  distinctly  at  one  time  than  at  another.  In  calm, 
dry  air  the  report  of  a  musket  may  be  heard  at  the  distance  of  five  miles, 
and  the  sound  of  cannon  has  been  heard  over  water  at  the  distance  of 
200  miles.  In  the  open  air  the  human  voice  may  be  heard  at  the  dis- 
tance of  230  yards  ;  and  Captain  Parry  informs  us  that  at  the  polar 
regions,  where  the  air  is  dense  and  dry,  a  conversation  may  be  carried 
on  between  two  persons  a  mile  apart.  The  explosions  of  the  volcano  of 
St.  Vincent  were  heard  at  Demarara,  a  distance  of  340  miles.  This  is 
the  greatest  distance  on  record  to  which  sound  has  been  conveyed  by  the 
atmosphere. 

Velocity  of  Sound. 

37.  When  a  gun  is  fired,  we  always  see  the  flash  before 
we  hear  the  sound.     Now,  we  see  the  flash  almost  instanta- 
neously ;  but  sound  requires  a  sensible  time  to  travel  over 
any  particular  distance.     In  ordinary  states  of  the  air  sound 
travels  at  the  rate  of  1120  feet  per  second. 

From  this  we  can  readily  calculate  the  distance  at  which  a  gun  is  fired 
when  we  know  the  interval  of  time  which  elapses  between  the  flash  and 
the  sound. 

Example.  Required  the  distance  at  which  a  gun  is  fired  when  the 
report  is  heard  three  seconds  after  the  flash  is  seen. 

Here,  distance  travelled  by  sound  in  1  second  =  1120  feet; 

Distance  travelled  in  3  seconds  =  3  times  1120  =  3360  feet ; 
which  gives  the  distance  required. 

In  the  same  way  we  can  find  the  distance  of  lightning  from  us,  by 
observing  the  number  of  seconds  which  elapse  between  the  flash  and  the 
sound  of  the  thunder.  As  the  human  pulse  very  nearly  beats  once  in 
every  second,  we  may  always  readily  find  the  interval  between  the  flash 
and  the  sound  by  counting  the  beats  of  the  pulse. 

38.  Solid  as  well  as  liquid  bodies  transmit  sound  even  bet- 
ter than  air. 

EXPERIMENTS. 

Exp.  1.  Strike  two  stones  together  under  water :  the  sound  will  be 
as  loud  as  if  they  had  been  struck  in  the  air. 

Exp.  2.  Scratch  the  end  of  a  log  of  timber  with  a  pin  :  a  person  with 
his  ear  at  the  opposite  end  will  distinctly  hear  the  sound. 

Exp.  3.   Place  the  end  of  a  long  iron  rod  between  the  teeth,  while 
the  other  end  rests  on  the  bottom  of  a  hollow  vessel :  a  whisper  uttered 
within  the  mouth  of  the  vessel  will  be  distinctly  heard,  though  it  would 
be  inaudible  through  the  air. 
11*   * 


126          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Exp.  4.  Suspend  a  poker  by  two  strings,  and  press  the  ends  one  in 
each  ear,  while  the  poker  is  allowed  to  hang  freely ;  strike  the  poker  :  a 
sound  like  the  tolling  of  a  large  bell  will  be  heard.  Take  the  ends  of 
the  strings  from  the  ear,  and  the  sound  will  be  comparatively  feeble. 

Reflection  of  Sound. 

39.  When  sound  strikes  against  any  fixed  surface,  it  is 
reflected  from  that  surface,  and  the  angle  of  reflection  is  equal 
to  the  angle  of  incidence.     This  law  of  reflection  is  the  same 
as  that  which  takes  place  with  respect  to  any  elastic  bodies. 

Thus,  for  example,  when  a  marble  is  thrown  obliquely  on  a  hard  pave- 
ment, it  is  reflected  from  the  surface  of  the  pavement  at  an  angle  equal 
to  the  incident  angle  —  that  is,  equal  to  the  angle  at  which  it  meets  the 
surface  of  the  pavement. 

Echoes. 

40.  Reflected  sounds  produce  echoes. 

In  order  that  an  echo  should  be  heard  distinct  from  the  sound  which 
produces  it,  the  reflecting  surface  must  be  at  such  a  distance  that  the 
reflected  sound  would  not  be  confounded  with  the  original  sound  pro- 
ceeding directly  to  the  ear.  Now,  the  human  ear  cannot  appreciate 
more  than  ten  separate  sounds  in  a  second ;  so  that,  in  order  that  two 
successive  sounds  may  be  distinctly  heard,  the  interval  between  them 
must  be  at  least  the  tenth  part  of  a  second ;  and,  since  sound  travels  at 
the  rate  of  1120  feet  per  second,  or  112  feet  in  the  tenth  part  of  a  sec- 
ond, it  follows  that  the  reflected  sound  must  travel  over  112  feet  more 
than  the  direct  sound  in  order  that  the  echo  of  a  single  sound  or  syllable 
may  be  distinctly  heard  from  the  sound  itself. 

Let  A  be  the  place  of  a  person 
giving  utterance  to  a  single  sound  ; 
C  the  place  of  the  person  who  hears 
the  echo ;  E  F  any  wall  or  obstacle 
•which  reflects  the  sound ;  A  B  the 
incident  sound;  B  C  the  reflected 
sound ;  B  D  a  line  perpendicular  to 
the  plane  of  the  wall :  then  the 
angle  A  B  D  is  called  the  angle  of 
incidence,  and  the  angle  D  B  C  that 
of  reflection.  Now,  the  sound  pro- 


Fig.  52. 


ducing  the  echo  travels  through  the  distance  A  B  added  to  B  C,  while 
the  direct  sound  simply  travels  through  the"  distance  A  C  ;  and,  there- 


PNEUMATICS. 


127 


fore,  if  the  former  distance  is  112  feet  greater  than  the  latter,  the  person 
at  C  would  distinctly  hear  the  echo  of  a  single  sound,  as  well  as  the 
sound  itself.  If  the  distance  ABC  exceeds  A  C  by  twice  112  feet,  the 
echo  of  two  distinct  sounds  may  be  heard,  as  in  the  case  of  the  word 
echo.  Some  echoes  repeat  several  syllables  in  succession.  There  are  also 
some  echoes  which  repeat  the  same  word  several  times ;  this  takes  place 
when  there  are  a  series  of  reflecting  surfaces  placed  at  different  distances 
from  the  speaker.  "When  a  hill  or  some  other  object  obstructs  the  direct 
sound,  the  echo  only  may  be  heard. 

Whispering   Galleries. 

41.  Sound,  as  well  as  light,  may  be  magnified  by  reflection ; 
it  is  on  this  principle  that  whispering  galleries  are  constructed. 

Let  A  C  B  E  represent  the 
wall  of  an  elliptical  building ; 
then  a  whisper  uttered  at  the  fo- 
cus a  will  be  distinctly  heard  at 
the  other  focus  b.  Here  the 
sound  proceeding  from  a  is  re- 
flected from  every  point  in  the 
wall  to  the  point  b  ;  for  example, 
the  sound  proceeding  along  the 
line  a  x  is  reflected  along  the  line 
x  b,  where,  from  the  property  of 
the  ellipse,  the  angle  a  x  y  of  the 
incident  sound  is  equal  to  the 
angle  s  x  b  of  the  reflected  sound ;  and  this  takes  place  for  every  point 
in  the  ellipse.  The  form  of  the  ellipse  is  peculiarly  adapted  for  a  whis- 
pering gallery,  not  only  on  account  of  the  property  just  mentioned,  but 
also  on  account  of  another  property,  viz. :  the  sum  of  the  lines  a  x  and 
b  x,  drawn  from  the  foci  a  and  b  to  any  point  in  the  ellipse,  is  always 
of  the  same  length ;  from  this  it  follows  that  the  various  reflected  sounds 
reach  the  ear  at  the  same  instant.  The  whispering  gallery  of  St.  Paul's 
Cathedral,  in  London,  depends  upon  a  similar  principle. 

42.  The  speaking  trumpet  is  much  used  at  sea  to  render  the  human 


Fig.  53. 


Fig.  54. 


128  NATURAL    AN1>    EXPERIMENTAL    PHILOSOPHY. 

voice  heard  at  a  great  distance.  When  a  word  is  spoken  at  A,  the  sound 
is  reflected  from  different  points  in.  the  interior  of  the  trumpet,  so  that 
the  sound  issues  from  the  wide  mouth  B  with  an  accumulated  force.  A 
strong  man's  voice,  with  a  good  instrument,  may  be  heard  at  the  distance 
of  three  miles. 

The  hearing  trumpet  is  very  useful  to  persons 
dull  of  hearing,  as  it  enables  them  to  hear  what  is 
spoken  to  them.  It  depends  upon  the  same  princi- 
ple as  the  speaking  trumpet.  The  aperture  A  is 
placed  within  the  ear  of  the  deaf  person,  and  the 
sound  emitted  at  B  is  concentrated  at  A  by  a  series 
of  reflections. 

43.  Further  Examples.  —  Almost  any  sound  produced  near  a  piano 
forte  whose  dampers  are  raised  finds  a  responsive  string. 

A  harp  or  guitar  in  a  room  \\ith  talking  company  is  often  mingling  a 
note  with  their  conversation. 

Savages  often  discover  the  approach  of  footsteps  by  applying  an  ear  to 
the  ground. 

Many  a  haunted  house,  so  called,  owes  its  reputation  to  some  innocent 
cause  which,  operating  without,  is  transmitted  to  the  apartments  within 
by  the  solid  walls,  and  interpreted  by  the  imagination  into  the  language 
of  ghosts.  Even  the  beating  of  one's  own  heart,  under  a  sense  of  fear, 
has  been  ascribed  to  a  trip  hammer  in  some  distant  machine  shop. 

The  resonance  of  a  room  is  irregular  and  indistinct  when  the  room 
contains  curtains,  carpets,  and  other  furniture,  or  a  crowded  assembly. 
Music  halls  have  generally  plain,  bare  walls. 

WINDS. 

44.  Currents  of  air,  or  winds,  are  produced  by  the  unequal 
distribution  of  heat  over  the  earth.     When  the  sun  shines 
over  any  particular  spot  on  the  earth,  the  air  immediately  over 
the  warm  ground  is  rarefied  by  the  heat,  and  consequently 
ascends,  while  the  surrounding  air,  being  cooler  and  heavier, 
rushes  in  to  supply  the  place  which  the  warm  air  has  left 
vacant.     In  order  to  show  the  truth  of  this  beautiful  law  of 
nature,  the  following  experiment  may  be  made  :  — 

Exp.  Make  a  wide  pasteboard  tube,  and  hold  it  in  an  inclined  position, 
with  its  upper  orifice  near  the  flame  of  a  candle  or  lamp ;  hold  a  lighted 
piece  of  paper  near  the  lower  orifice  of  the  tube,  and  blow  it  out ;  the 
smoke  from  the  paper  is  drawn  up  the  tube,  and  rises  with  the  ascending 
current  of  air  proceeding  from  the  candle. 


PNEUMATICS.  129 

The  land  and  sea  breezes,  which  chiefly  occur  in  warm  countries,  afford 
a  simple  and  an  instructive  illustration  of  the  manner  in  which  winds 
are  generally  produced.  These  winds  blow  from  the  sea  to  the  land 
during  the  day,  and  the  reverse  of  this  takes  place  during  the  night.  In 
the  daytime  the  land  becomes  more  heated  than  the  water,  and  thus 
the  air  over  the  land,  becoming  rarefied,  ascends  ;  while  the  cool,  dense 
air  over  the  water  rushes  in  to  supply  the  place  of  the  rarefied  air.  On 
the  contrary,  during  the  night  the  land  loses  its  heat  more  rapidly  than 
the  water,  until  at  length  it  becomes  cooler  than  the  water  ;  in  this  case, 
therefore,  a  current  of  air  sets  in  from  the  land  to  the  water.  These 
winds  tend  to  equalize  the  temperature  of  islands  and  all  places  on  the 
sea  coast. 

45.  Winds  are  divided  into  three  kinds :  these  are,  the 
constant  winds,  which  always  blow  in  ihe  same  direction  ;  the 
monsoons,  or  those  which  blow  one  half  of  the  year  in  one 
direction,  and  the  other  half  in  the  contrary  direction ;  the 
variable  winds,  which  do  not  appear  to  follow  any  regular  law. 

(I.")  THE  CONSTANT  WINDS,  which  are  also  called  TRADE  WINDS,  extend 
within  30  degrees  on  each  side  of  the  equator.  The  cause  of  these  winds 
may  be  explained  in  the  following  manner  :  The  equatorial  portion  of 
the  earth  being  the  hottest,  the  cool  air  from  the  temperate  and  polar 
regions  rushes  towards  the  equator  to  supply  the  place  of  the  heated  air, 
which  is  there  constantly  ascending  ;  and  if  the  earth  were  not  to  turn 
on  its  axis,  this  would  occasion  two  winds,  one  blowing  directly  from  the 
north  pole  towards  the  equator  in  the  northern  hemisphere,  and  the  other 
blowing  from  the  south  pole  to  the  equator  in  the  southern  hemisphere  ; 
but  as  the  earth  revolves  from  west  to  east,  the  air  towards  the  poles  has 
a  less  rotatory  motion  than  the  solid  parts  of  the  earth  at  the  equator  ;  it 
consequently  follows,  that  when  this  air  arrives  at  the  equator  it  does  not 
move  with  the  same  speed  as  the  earth,  and  thus  a  wind  blowing  from 
the  east  (contrary  to  the  earth's  motion)  is  produced  at  the  equator. 

Now,  the  air  within  the  northern  tropic,  before  it  reaches  the  equator, 
has  a  twofold  motion  —  that  is  to  say,  it  has  its  original  motion  from 
north  to  south,  and,  owing  to  the  earth's  rotation,  it  has  relatively  a  mo- 
tion from  east  to  west ;  but  as  these  motions  take  place  at  the  same  time, 
it  causes  the  wind  to  blow  from  the  north-east,  which  is  the  direction  of 
the  trade  in  the  northern  tropic.  Reasoning  in  the  same  manner,  it 
follows  that  the  trade  wind  in  the  southern  tropic  must  blow  from  the 
south-east. 

These  winds  tend  to  equalize  the  temperature  of  the  globe,  and  to 
maintain  the  purity  of  the  atmosphere ;  for  while  the  cool  air  of  the 


130  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

polar  and  temperate  regions  is  constantly  descending  towards  the  torrid 
zone,  the  warm  air  of  this  zone  is  constantly  ascending  and  moving 
towards  the  polar  and  temperate  region  of  the  earth ;  thus  while  the 
cool  air  of  the  frigid  and  temperate  zones  moderates  the  excessive  heat 
of  the  torrid  zone,  at  the  same  time  the  warm  air  of  the  latter  elevates 
the  temperature  of  the  former. 

If  the  earth  within  the  tropics  were  covered  with  water,  the  trade 
winds  would  regularly  blow  in  the  manner  just  described  ;  but  owing  to 
the  unequal  distribution  of  land  and  water,  these  winds  are  subject  to 
certain  remarkable  deviations.  As  might  have  been  expected,  the  trade 
winds  blow  with  the  greatest  regularity  over  the  expanse  of  the  Pacific 
Ocean. 

(2.)  Moxsooxs.  —  When  a  trade  wind  is  turned  back  or  diverted  by 
overheated  districts  from  its  regular  course  at  stated  seasons  of  the  year, 
it  is  regarded  as  a  monsoon.  Thus  the  African  monsoons  of  the  Atlantic, 
the  monsoons  of  the  Gulf  of  Mexico;  and  the  Central  American  mon- 
soons of  the  Pacific  are,  for  the  most  part,  formed  of  the  north-east  trade 
winds,  which  are  turned  back  to  restore  the  equilibrium  which  the  over- 
heated plains  of  Africa,  Utah,  Texas,  and  New  Mexico  have  disturbed. 

"When  the  monsoons  prevail  for  five  months  at  a  time,  (for  it  takes 
about  a  month  for  them  to  change  and  become  settled,)  then  both  they 
and  the  trade  winds,  of  which  they  are  formed,  are  called  monsoons. 
The  south-west  and  north-east  monsoons  of  the  Indian  Ocean  afford  an 
example  of  this  kind.  The  south-west  monsoons  of  the  Indian  Ocean 
blow  from  May  to  September  inclusive.  They  are  caused  by  the  intense 
heat  which  the  rays  of  a  cloudless  sun  produce  during  the  summer  time 
upon  the  Desert  of  Gobi  and  the  burning  plains  of  Central  Asia.  When 
the  sun  is  north  of  the  equator,  the  force  of  his  rays,  beating  down  upon 
these  wide  and  thirsty  plains,  causes  the  air  to  expand  and  ascend. 
There  is,  consequently,  a  rush  of  air,  especially  from  towards  the  equa- 
tor, to  restore  the  equilibrium  ;  and  in  this  case  the  force  which  tends  to 
draw  the  north-east  trade  winds  back  becomes  greater  than  the  force 
which  is  acting  to  drive  them  forward. 

When  it  is  summer  time  in  Africa  south  of  the  .equator,  the  winds  are 
blowing  from  the  north-east,  in  obedience  to  the  trade  wind  force,  which 
prevails  from  November  to  March  inclusive  ;  hence  we  have  the  north- 
east monsoons.  The  monsoon  season  may  always  be  known  by  referring 
to  the  cause  which  produces  these  winds.  Thus,  by  recollecting  where 
the  dry  and  overheated  plains  are,  we  know  at  once  that  these  winds  are 
rushing  with  greatest  force  towards  these  plains  at  the  time  of  their  hot- 
test season  of  the  year. 

These  winds  are  of  considerable  importance  to  navigators  who  make 
voyages  to  the  East  Indies. 


PNEUMATICS.  131 

(3.)  THE  VAKIABLE  WINDS  blow  at  all  places  at  a  distance  from  the 
equator.  The  variableness  depends  upon  a  variety  of  causes  ;  for  what- 
ever tends  to  disturb  the  equilibrium  of  the  atmosphere  will  occasion  a 
change  in  the  direction  of  the  wind. 

The  cold  air  of  the  polar  regions  is  constantly  flowing  towards  the 
warmer  regions,  partly  as  an  upper  current,  according  to  the  general 
law  of  atmospheric  circulation,  and  partly  as  a  surface  wind  :  hence  in 
the  northern  hemisphere  we  have  a  prevalence  of  north-east  winds. 
These  winds,  finding  an  open  path  in  North  America,  from  one  end  of 
the  continent  to  the  other,  sweep  from  the  borders  of  the  Arctic  Ocean 
as  far  as  the  Gulf  of  Mexico.  They  strike  obliquely  against  the  Rocky 
Mountains,  run  along  their  slopes,  and,  being  reflected  by  this  high  chain, 
descend  as  a  north-west  wind  into  the  valley  of  the  Mississippi,  accom- 
panied by  cold  and  storms.  Proceeding  towards  the  Atlantic  coast,  they 
meet  the  south-west  or  the  equatorial  winds. 

This  conflict  between  the  polar  and  equatorial  winds,  so  opposite  in 
character  and  direction,  gives  to  our  climate  one  of  its  most  remarkable 
features,  —  that  of  changeableness,  —  that  great  variety  of  temperature, 
of  drought  and  of  humidity,  of  fair  weather  and  foul,  which  mark  the 
seasons  with  uncertainty,  and  the  labors  of  the  husbandman  with  doubt- 
ful results. 

Sirocco  and   Simoom. 

There  are  certain  winds  which  have  received  peculiar  names,  such  as 
the  Sirocco  and  the  Simoom.  These  winds  are  injurious  to  life,  on  ac- 
count of  the  burning  sands,  or  on  account  of  the  pestilential  swamps, 
over  which  they  blow. 

The  Sirocco  blows  from  Africa  over  the  south  of  Europe ;  it  is  es- 
pecially felt  in  the  south  of  Italy  and  Spain.  During  the  continuance 
of  this  hot  wind,  the  vegetable  creation  loses  its  freshness  and  beauty, 
and  the  animals  of  the  field,  as  well  as  man,  appear  to  languish  and 
droop  with  excessive  exhaustion. 

The  Simoom,  which  blows  over  the  burning  deserts  of  Africa  and  Asia, 
is  of  all  other  winds  the  most  destructive  to  life.  •  The  breathing  of  this 
wind  sometimes  occasions  instantaneous  death  ;  and  to  save  their  lives, 
travellers  usually  throw  themselves  down  with  their  faces  on  the  ground, 
until  the  desolating  wind  has  passed  over  them. 

46.  Winds  receive  names  according  to  the  rate  at  which  they  blow. 
When  a  wind  blows  at  the  rate  of  o  miles  an  hour,  it  is  called  a  gentle 
breeze ;  at  10  miles  an  hour,  a  brisk  gale  ;  at  40  miles  an  hour,  a  high 
wind  or  storm  ;  and  at  80  miles  an  hour,  a  hurricane.  The  force  of  the 
wind  increases  with  the  square  of  the  velocity :  thus  a  hurricane,  having 


132          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

8  times  the  velocity  of  a  brisk  gale,  would  strike  trees  and  houses  with 
64  times  the  force  that  a  brisk  gale  would  do. 

BALLOONS. 

47.  Balloons  rise  in  the  atmosphere  in  the  same  manner  as  smoke  as- 
cends, or  as  a  cork  rises  in  water.    A  soap  bubble  is  a  little  balloon  in- 
flated with  the  warm  air  from  the  lungs ;  and  it  ascends  because  it  is 
specifically  lighter  than  the  surrounding  air.     A  balloon  is  sometimes 
made  of  thin  paper  ;  the  air  which  it  contains  is  rarefied  by  means  of  the 
flame  of  a  piece  of  sponge  dipped  in  spirits  of  wine  and  placed  beneath 
an  opening  made  in  the  under  part  of  the  balloon.     This  kind  of  balloon 
was  invented  by  Montgolfier. 

From  its  extreme  lightness,  hydrogen  is  better  fitted  than  any  other 
substance  to  inflate  balloons  ;  though  coal  gas,  from  its  greater  cheapness, 
is  generally  used.  Very  large  gas  balloons,  of  a  pear-like  shape,  are 
made  of  oiled  silk,  and  inflated  with  common  street  gas,  which  is  con- 
siderably lighter  than  atmospheric  air.  The  balloon,  being  filled  with  this 
light  gas,  is  rendered  specifically  lighter  than  the  air  ;  it  therefore  ascends 
until  it  arrives  at  an  elevation  where  the  surrounding  air  and  the  balloon 
have  the  same  specific  gravity.  The  car  which  bears  the  aeronaut  is 
supported  by  network  which  goes  over  the  body  of  the  balloon.  When 
the  aeronaut  wishes  to  descend,  he  pulls  a  cord  which  opens  a  valve  at 
the  top  of  the  balloon,  and  thus  allows  a  portion  of  the  light  gas  to 
escape,  and  thereby  renders  the  balloon  specifically  heavier  than  the  air. 

The  buoyancy,  or  ascending  force,  of  a  balloon  may  be  easily  calculated. 
Suppose  the  balloon  to  contain  32,000  cubic  feet  of  gas,  the  weight  of 
each  cubic  foot  of  air  to  be  1  y\y  ounces,  and  the  weight  of  each  cubic 
foot  of  gas  to  be  1  ounce ;  then  each  cubic  foot  of  the  balloon  will  have 
a  buoyancy  of  j-\j-  of  an  ounce,  and  the  whole  balloon  will  have  a  buoy- 
ancy of  3200  ounces,  or  200  Ibs.  If  the  weight  of  the  car  and  the 
material  of  the  balloon  be  60  Ibs.,  then  the  balloon  will  ascend  when  the 
weight  of  the  aeronaut  does  not  exceed  140  Ibs. 

48.  Additional  Facts.  — The  pressure  of  the  atmosphere  at  the  surface 
of  the  earth  keeps  a  certain  quantity  of  air  in  combination  with  water, 
so  as  to  form  part  of  the  liquid  mass.     The  air  reappears  at  once  on 
taking  off  the  pressure.     This  admixture  of  air  in  water  is  necessary  to 
the  life  of  fishes. 

A  balloon  which  is  only  half  full  at  the  surface  of  the  earth  becomes 
quite  full  when  it  has  risen  3^  miles,  because  at  that  height  air  from  be- 
low doubles  its  volume,  on  account  of  the  diminished  pressure. 

The  downy  seeds  of  plants  seen  floating  about  upon  the  winds  of 
autumn  are  not  lighter  than  air,  but  have  so  much  bulk  and  surface  in 


PNEUMATICS.  133 

proportion  to  their  weight,  that  the  friction  upon  them  of  the  moving  air 
is  greater  than  their  weight,  and  carries  them  along. 

Smoke  consists  of  the  dust  and  visible  particles  which  are  separated 
from  the  fuel  without  being  burned,  and  light  enough  to  be  carried  aloft 
by  the  rising  current  of  heated  air  ;  but  all  that  is  visible  of  smoke  is 
really  heavier  than  ah*,  and  soon  falls  again. 

When  a  low  house  adjoins  a  lofty  one,  the  wind  blowing  towards  the 
latter  is  obstructed  ;  and  if  the  top  of  a  low  chimney  be  there,  the  com- 
pressed air  enters  it  and  pours  downwards.  Again,  whenever  from  the 
nature  of  buildings  eddies  of  wind  occur,  or  unequal  pressures,  as  at 
street  corners,  &c.,  the  chimneys  around  do  not  act  regularly. 


EXERCISES  ON  PNEUMATICS. 

1.  If  100  cubic  inches  of  atmospheric  air  weigh  30  grains,  what  will  be 
the  weight  of  1  cubic  foot?    (See  Art.  5.)  Ans.  1.08  oz. 

2.  When  the  elevation  of  the  mercury  in  the  barometer  is  28  inches, 
what  will  be  the  height  of  a  column  of  water  supported  by  the  atmos- 
pheric pressure  ?    (See  Art.  8.) 

Column  of  mercury  supported  by  the  atmosphere  =  28  in. ; 

"         water  ««  «  "         =  13£  X  28  in.  ? 

=  31i  feet. 

3.  Required  the  same  as  in  the  last  example,  when  the  elevation  of  the 
mercury  is  24  inches  ?  Ans.  27  feet. 

4.  Allowing  that  a  cubic  inch  of  mercury  weighs  £  lb.,  what  would 
be  the  pressure  of  the  atmosphere  at  the  top  of  a  mountain,  where  the 
mercury  in  the  barometer  stands  at  the  height  of  20  inches  ? 

Ans.  10  Ibs.  per  sq.  in. 

5.  A  given  portion  of  air  has  a  pressure  of  15  Ibs.  per  square  inch 
when  its  volume  is  5  cubic  feet :  what  will  be  its  elasticity  when  it  is 
compressed  into  the  space  of  3  cubic  feet  ?     (See  Art.  16.) 

Pressure  when 'its  vol.  is  5  c.  ft.  =  15  Ibs. ; 
"  "       vol.  is  1  c.  ft.  =  5  X  15  Ibs. ; 

«  «       vol.  is  3  c.  ft.  =  5  X  15=  25  Ibs. 

6.  Required  the  same  as  in  the  last  example,  when  the  original  pres- 
sure is  20  Ibs.  with  a  volume  of  2  cubic  feet,  and  the  new  volume  is  5 
cubic  feet  ?  Ans.  8  Ibs. 

7.  If  the  column  of  mercury  in  the  gauge  of  the  air  pump  (see  Art. 
19)  stands  at  27  inches,  when  the  mercury  in  the  barometer  is  30  inches, 
what  is  the  elasticity  of  the  air  in  the  receiver,  as  compared  with  the 
external  air  ?  Ans.  j1-. 

12 


134          NATURAL   AND   EXPERIMENTAL    PHILOSOPHY. 

8.  If  the  section  of  the  Magdebourg  hemispheres  (see  Exp.  3,  Art.  20) 
contains  6  square  inches,  and  it  requires  a  weight  of  87  Ibs.  to  separate 
them,  what  is  the  pressure  of  the  external  air  upon  each  square  inch  ? 

Ans.  14£  Ibs. 

9.  To  what  height  may  water  be  raised  by  the  common  pump,  at  a 
place  where  the  barometer  stands  at  24  inches  ? 

jins.  27  feet.    (See  Art.  21.) 

10.  In  the  hydrostatic  press,  (see  Art.  26,)  if  the  large  piston  P  con- 
tains 18  square  inches,  the  small  one  l£  square  inches,  and  if  the  ad- 
vantage gained  by  the  lever  G  H  is  6,  what  will  be  the  pressure  exerted 
upon  the  press  board,  when  a  pressure  of  60  Ibs.  is  applied  to  the  handle  ? 

Here  the  whole  pressure  on  the  small  piston  is  6  times  60  Ibs.,  or  360 
Ibs. ;  that  is, 

Pressure  on  l£  sq.  in.  =  360  Ibs. ; 

18  sq.  in.  =  12  times  360  Ibs.  =  4320  Ibs. ; 
which  gives  the  upward  pressure  on  the  large  piston. 

11.  Required  the  same  as  in  the  last  example,  when  the  large  piston 
contains  20  square  inches,  the  small  one  2«quare  inches,  the  pressure  on 
the  handle  being  40  Ibs.,  and  the  advantage  gained  by  the  lever  8  ? 

Ans.  3200  Ibs. 

12.  Required  the  distance  of  lightning  when  the  flash  is  seen  9  sec- 
onds before  the  thunder  is  heard  ?     (See  Art.  37.) 

Ans.  1  mile  600  yards. 

13.  At  what  distance  must  a  gun  be  fired  so  that  the  interval  between 
the  flash  and  the  sound  may  be  3£  seconds  ?  Ans.  3020  feet. 


LIGHT  AND  HEAT. 

LIGHT. 

1.  LIGHT  renders  the  objects  in  the  external  world  visible 
to  us :  the  sense  of  sight  is  the  eye ;  the  rays  of  light,  pro- 
ceeding from  surrounding  objects,  enter  the  eye,  that  wonder- 
ful optical  instrument,  and  by  acting  upon  the  optic  nerve, 
produce  the  sensation  of  vision. 

2.  Light  emanates  from  all  luminous  bodies  —  such  as  the 
sun,  the  stars,  and  substances  in  a  state  of  combustion. 

The  sun  is  the  great  source  of  light  as  well  as  of  heat ;  but  there  are 
many  other  sources  of  light,  such  as  —  (1.)  Chemical  light,  or  that  which 
is  derived  by  chemical  action ;  (2.)  Electric  light,  or  that  light  which  is 
evolved  by  the  electric  spark  ;  (3.)  Light  of  friction,  that  which  is  ob- 
tained by  striking  two  dissimilar  hard  bodies  together ;  (4.)  Phospho- 
rescent light,  or  that  light  which  is  emitted  by  certain  bodies  at  the  ordi- 
nary temperature  of  the  air. 

3.  Non-luminous  bodies  become  visible  to  us  only  by  re- 
flecting the  light  which  falls  upon  them  from  some  luminous 
body. 

The  fixed  stars,  as  well  as  the  sun,  shine  by  their  own  light ;  but  the 
moon  and  the  planets,  with  their  satellites,  shine  only  by  reflected  light. 

4.  Bodies  are  divided  into  transparent  and  opaque.    Trans- 
parent bodies,  such  as  glass,  allow  the  rays  of  light  to  pass 
freely  through  them ;   in  other  words,  we  can  see  objects 
through  them.     Opaque  bodies,  such  as  a  sheet  of  tin,  do  not 
allow  the  rays  of  light  to  pass  through  them. 

5.  Light  travels  at  an  inconceivably  rapid  rate. 

It  takes  about  eight  minutes  in  travelling  from  the  sun  to  us,  that  is, 
it  moves  at  the  rate  of  about  192,000  miles  per  second.  As  regards  all 
phenomena  upon  earth,  they  may  be  considered  as  happening  at  the  very 
instant  when  the  eye  perceives  them ;  the  difference  of  time  being  too 
small  to  be  appreciated. 

(135) 


136 


NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 


Light  proceeds  from  visible  objects  in  straight  lines. 

Thus  we  can  only  see  a  body,  when  looking  through  a  straight  tube, 
by  directing  the  tube  towards  the  body. 

6.  The  intensity  of  light  varies  with  its  distance  from  us  ; 
that  is  to  say,  like  all  other  principles  which  emanate  from  a 
centre,  the  intensity  of  light  decreases  as  the  squares  of  the 
distances  increase. 

Thus,  at  the  distance  of  two  yards,  the  intensity  of  light  will  be  .one 
fourth  of  what  it  is  at  one  yard ;  at  three  yards,  the  intensity  will  be 
one  ninth  of  what  it  is  at  one  yard ;  and  so  on :  the  reason  of  this  is 
rendered  manifest  by  Fig.  1. 


Fig.  1. 

7.  There  are  two  remarkable  laws  of  light,  viz.,  reflection 
and  refraction. 

Every  boy  is  familiar  with  the  reflection  of  the  sunlight  from  a  bit 
of  looking  glass. 

When  we  look  upon  the  surface  of  a  stream,  we  see  the  objects  on  its 
opposite  bank  reflected  from  its  surface.  In  this  case,  the  reflected  im- 
ages of  the  objects  appear  turned  upside  down. 

We  see  our  faces  reflected  from  the  surface  of  the  looking  glass.  In 
this  case,  the  side  of  our  face  to  the  left  appears  on  the  right  of  the  re- 
flected image. 

We  plunge  a  stick  into  water  ;  the  stick  appears  bent :  this  is  owing 
to  refraction. 

We  look  through  a  tumbler  of  water  upon  an  object  placed  on  the 
opposite  side  ;  the  object  appears  very  much  enlarged  and  somewhat  dis- 
torted :  this  is  owing  to  refraction  :  the  rays  of  light  proceeding  from  the 
object  are  changed  from  their  right-lined  course  in  passing  through  the 
transparent  medium.  This  change  of  direction  is  called  the  effect  of 
refraction. 

8.  The  great  Newton  considered  that  light  is  an  emanation  from 


LIGHT   AND   HEAT. 


137 


luminous  bodies,  and  consists  of  very  minute  particles  which  are  too  fine 
or  subtile  to  exhibit  the  ordinary  properties  of  matter,  and  which  travel 
in  straight  lines  with  inconceivable  velocity,  and  produce  the  sensation 


Fig.  2. 

of  light  by  passing  into  the  eye,  striking  against  the  expanded  nerve  of 
vision  called  the  retina.  This  has  been  called  the  corpuscular  theory  of 
light.  There  is  another  theory  of  light,  pretty  generally  now  adopted 
by  philosophers,  which  is  known  as  the  undulatory  theory  of  light :  ac- 
cording to  this  theory  light  is  supposed  to  be  propagated  by  the  undula- 
tions of  a  subtile  ethereal  medium  which  pervades  all  space.  Now,  as 
either  of  these  theories  serves  the  purpose  of  explaining  and  classifying 
the  facts  which  we  shall  have  occasion  to  notice,  we  shall  not  limit  our- 
selves by  decidedly  adopting  either  the  one  theory  or  the  other. 
12* 


138 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


EXPERIMENTS    ELUCIDATING   THE   LEADING    PRINCI- 
PLES OF   OPTICS. 

9.    REFLECTION. 

Exp.  1.  Lay  a  small  looking  glass  upon  the  floor,  with  its  face  upper- 
most ;  place  a  burning  candle  on  the  floor,  at  some  distance  from  the 
looking  glass,  so  that  the  light  from  the  candle  may  fall  obliquely  upon 
the  surface  of  the  glass :  place  yourself  on  the  side  opposite  to  the  can- 


die,  shifting  your  position  until  you  catch  a  sight  of  the  image  of  the 
candle  reflected  from  the  glass. 

(1.)  The  reflected  ray  makes  the  same  angle  with  the  glass  as  the  inci- 
dent ray. 

(2.)  The  image  of  the  candle  appears  as  much  below  the  surface  of 
the  glass  as  the  candle  itself  stands  above  the  glass. 

Bring  the  candle  about  a  foot  nearer  to  the  reflector :  then  you  will 
have  to  bring  your  eye  the  same  distance  nearer  to  it ;  thereby  showing 
that  the  reflected  ray  always  makes  the  same  angle  with  the  surface  of 
the  reflector  that  the  incident  ray  does. 

The  angle  of  reflection  is  always  equal  to  the  angle  of  inci- 
dence. 

Thus,  let  E  F  be  the  reflecting  surface, 
A  B  the  incident  ray,  B  C  the  reflected 
ray,  and  B  P  a  perpendicular  to  the  sur- 
face E  F ;  then  the  angle  A  B  P  which 
the  incident  ray  makes  with  this  perpen- 
dicular is  called  the  angle  of  incidence, 
and  the  angle  GBP  which  the  reflected 
ray  makes  with  this  perpendicular  the 


Fig.  4. 


LIGHT   AND    HEAT.  139 

angle  of  reflection ;  and  these  two  angles  are  always  equal  to  each 
other. 

It  is  quite  true,  at  the  same  time,  that  the  angle  ABE,  which  the 
incident  ray  makes  with  the  surface  of  fhe  reflector,  is  always  equal  to 
the  angle  F  B  C  which  the  reflected  ray  makes  with  the  surface  of  the 
reflector. 

In  Fig.  5,  A  B  is  the  incident  ray  proceeding  from  the  top  of  the  tree, 
and  B  C  the  reflected  ray ;  E  F  is  the  incident  ray  proceeding  from  one 
of  the  lower  branches,  and  F  C  the  reflected  ray.  Now,  because  of  the 


Fig.  5. 

equality  of  the  angles  of  incidence  and  reflection,  the  image  D  of  the  top 
of  the  tree  appears  as  much  below  the  surface  of  the  water  as  the  top 
of  the  tree  A  is  really  above  it. 

Exp.  2.  To  obtain  three  or  more  reflected  images  of  an  object.  —  Place 
a  small  mirror  perpendicular  to  a  larger  one :  put  any  object  between 
them ;  bring  your  eyes  in  front  of  the  smaller  mirror ;  you  will  distinctly 
see  three  reflected  images  of  the  object  —  that  is,  one  image  from  the 
reflection  of  the  large  mirror,  another  from  the  reflection  of  the  small 
mirror,  and  a  third  from  the  double  reflection  of  the  two  mirrors. 

Place  yourself  before  two  large,  parallel  looking  glasses,  fixed  to  the 
opposite  walls  of  a  room,  and  you  will  see  a  countless  series  of  images 
reflected  from  the  glass.  This  effect  is  produced  by  a  series  of  successive 
reflections. 

Exp.  3.  To  get  a  sight  of  the  back  of  your  head.  —  Place  yourself  with 
your  back  towards  a  large  looking-glass ;  hold  a  small  looking  glass  with 
its  face  towards  you,  but  a  little  to  one  side,  and  with  its  surface  somewhat 
inclined  to  the  plane  of  the  large  mirror ;  after  a  little  adjustment  you 
will  get  a  distinct  view  of  the  back  portion  of  your  head.  It  is  scarcely 


140         NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


necessary  to  say  that  this  is  due  to  two  reflections :  the  back  part  of  your 
head  is  reflected  from  the  large  mirror  to  the  small  one,  and  then  this 
last  image  is  reflected  from  the  surface  of  the  small  mirror  to  the  eye. 

Exp.  4.    A  person  may  see  his  whole  figure  reflected  from  the  surface 
of  a,  comparatively  small  mirror.  —  Place  yourself  a  few  feet  in  front  of 
a  mirror  A  B ;  move  backwards  until 
you  get  a  sight  of  your  whole  figure : 
the  rays  of  light  C  A  proceeding  from 
your  head  fall  perpendicularly  upon 
the  mirror,  and  are  therefore  reflected 
back  in  the  same  line ;  the  rays  B  D 
proceeding  from  your  feet  fall  obliquely 
upon  the  mirror,  and  are  therefore  re- 
flected according  to  the  law  of  reflection  before  explained,  in  the  direction 
B  C ;  so  the  image  of  the  image  of  the  feet  is  seen  at  F  in  the  direction 
of  the  line  C  B  produced.     The  image,  as  before  stated,  will  appear  to  be 
standing  as  much  behind  the  glass  as  the  actual  figure  is  standing  before  it. 

Exp.  5.     The  magic  perspective,  as  it  is  called,  is  produced  by  an  ar- 
rangement of  reflectors  which  enables  a  person  to  see  an  object  notwith- 


Fig.  6. 


Fig.  7. 

standing  the  interposition  of  an  opaque  screen.  I,  h,  g,  and  k  are  look- 
ing glasses  inclined  at  angles  of  45°  to  the  horizon ;  P  is  the  object,  P  I 
the  incident  rays,  and  I  h  g  k  A  the  course  of  the  reflected  rays ;  and 
the  eye  at  A  perceives  the  image  of  the  object  in  the  last  mirror  in  the 
direction  A  k. 

Exp.  6.  The  kaleidoscope  consists  of  a  long  tube,  blackened  inside, 
having  three  pieces  of  looking  glass  inserted  in  it  lengthwise ;  one  end 
of  the  tube  is  closed  by  a  piece  of  window  glass,  and  on  it  is  fitted  a  con- 
tinuation of  the  tube,  the  end  of  this  tube  being  closed  with  a  piece  of 
ground  window  glass  ;  the  space  between  these  two  glasses,  which  does 
not  exceed  a  quarter  of  an  inch,  is  filled  with  pieces  of  colored  glass  ;  the 
other  extremity  is  covered  with  a  cap  having  a  small  hole  through  its 
centre,  to  which  the  eye  must  be  applied.  Hold  the  tube  with  the  ground 
glass  to  the  light ;  look  through  the  hole  at  the  pieces  of  colored  glass, 
and  the  image  of  a  regular  hexagonal  star  will  be  seen.  The  effect  is 
due  to  the  successive  reflections  which  take  place  from  the  surfaces  of 


LIGHT   AND    HEAT. 


141 


the  three  mirrors ;  the  rays  proceeding  from  each  bit  of  glass  undergo 
five  reflections,  which,  with  the  object  itself,  presents  six  distinct  images 
to  the  eye. 

Exp.  7.  A  concave  mirror.  —  Look  into  the  concave  face  of  a  watch 
glass,  taking  care  to  have  a  dark  ground  behind  it ;  -after  adjusting  for 
the  focus,  you  will  see  a  small  inverted  image  of  your  face  reflected 
from  the  concave  surface  of  the  glass. 

Hold  a  small  object,  such  as  the  head  of  a  pin,  very  near  to  the  sur- 
face of  the  reflector  :  an  enlarged  image  of  the  object  will  be  seen  behind 
the  glass. 

A  convex  mirror.  —  Look  at  the  convex  face  of  the  watch  glass,  and 
similar  effects  will  be  observed. 


10.    REFRACTION. 

Exp.  1.    To  form  the  image  of  a  candle  by  the  transmission  of  its  light 
through  a  hole.  —  Pierce  a  thick  sheet  of  writing  paper  with  a  stout 


darning  needle;  hold  the  paper  between  a  lighted  candle  and  the  wall 
of  the  room :  an  inverted  image  of  the  candle  will  be  thrown  upon  the 
wall ;  move  the  paper  forwards  or  backwards,  until  you  have  attained 
that  position  which  gives  the  most  distinct  image  :  you  have  then  got 
the  focus. 

This  effect  is  simply  due  to  the  principle  that  light  is  propagated  in 
straight  lines.  The  rays  of  light  proceeding  from  the  object  cross  one 
another  in  passing  through  the  orifice  O  ;  the  rays  from  the  top  A  of  the 
candle,  pursuing  the  straight  line  A  O  B,  fall  upon  the  wall  at  B  ;  and, 
in  like  manner,  the  rays  from  the  bottom  C  of  the  candle  fall  upon  the 
wall  at  D,  thereby  producing  an  inverted  image  of  the  candle  upon 
the  wall. 

Bring  the  candle  nearer  to  the  wall,  and  at  the  same  time  shift  the 
position  of  the  orifice  so  as  to  adjust  the  focus,  and  you  obtain  an  image 
of  smaller  size,  but  of  more  intensity. 

Exp.  2.  Place  an  empty  vessel  so  as  to  make  the  shadow  of  its  edge 


142         NATURAL  AND   EXPERIMENTAL  PHILOSOPHY. 


to  fall  exactly  at  the  lower  angle  b  —  that  is,  so 
that  the  ray  of  light  proceeding  "from  the  candle 
shall  be  in  the  direction  s  d  b ;  now  fill  the  vessel 
with  water,  and  the  light  from  the  refraction  of 
the  liquid  will  be  extended  over  the  bottom  of 
the  vessel,  as  shown  in  Fig.  9. 

Exp.  3.    Place  a  coin  C  at  the  bottom  of  an 
empty  tumbler ;  bring  your  eye  E  in  a  line  E  K  G  with  the  edge  of  the 
glass  and  the  outer  edge  of  the  coin ;  without 
moving  your  eye,  pour  water  into  the  glass : 
the  wh'ole  of  the  coin  will  now  be  visible  — 
that  is,  it  will  be  seen  in  the  direction  E  D  B. 
The  ray  C  D  upon  passing  out  of  the  water 
becomes  bent  in  the  direction  D  E,  so  that  the 
edge  C  is  now  seen  in  the  direction  E  D  B. 

When  a  ray  of  light  is  thus  bent  from  its 
straight-lined  course,  it  is  said  to  undergo  re- 


fraction. 


Fig.  10. 


Light  always  undergoes  refraction  when  it  passes  obliquely 
from  one  medium  to  another.  But  when  the  light  passes 
perpendicularly  from  the  surface  of  the  one  medium  to  that 
of  the  other,  it  is  not  altered  in  its  straight-lined  course. 

Exp.  4.  Fill  a  tumbler  with  water  ;  place  the  leaf  of  a  book  close  to 
one  side  of  the  glass ;  look  through  the  water  at  the  print ;  you  will  see 
it  much  enlarged.  Here  the  round  portion  of  the  glass-  acts  as  a  convex 
lens,  which,  on  the  principle  of  refraction,  causes  the  print  to  appear 
larger  than  it  really  is. 

A  cylindrical  bottle  filled  with  water,  used  in  this  manner,  makes  a 
good  microscope. 

A  good  reading  glass  may  be  formed  by 
crossing  two  bottles  filled  with  clear  water,  as 
shown  in  Fig.  11,  and  looking  through  the 
crossed  portion. 

Exp.  5.  Take  one  of  the  convex  lenses  of  an 
aged  person's  spectacles,  and  hold  it  between  a 
candle  and  the  wall  of  the  room ;  an  inverted 
image  of  the  candle  will  be  thrown  upon  the 
wall;  move  the  lens  forwards  or  backwards 
until  you  have  attained  that  position  which 
gives  the  most  distinct  image ;  you  have  then  got  the  focus  of  the  lens. 

Here  the  central  rays  c  F  C  pass  perpendicularly  through  the  lens, 


Fig.  11. 


LIGHT   AND    HEAT.  143 


Fig.  12. 

and  undergo  no  refraction  ;  the  extreme  rays  a  D,  falling  obliquely 
upon  the  lens,  are  bent  in  passing  through  it,  and  are  thus  bent  or  re- 
fracted into  the  course  D  A ;  in  like  manner,  b  E  is  refracted  into  the 
direction  E  B  ;  and  hence  the  image  of  the  candle  appears  inverted. 

Exp.  6.  Look  through  the  spectacle  eye  upon  some  print ;  move  the 
lens  up  or  down  until  you  have  got  the  focus ;  the  print  will  appear 
considerably  enlarged. 

Perform  the  same  experiment  with  the  lens  of  the  spectacles  used  by 
a  short-sighted  person  ;  the  print  will  appear  smaller  than  it  really  is. 
In  this  case,  the  lens  is  hollow  or  concave. 

Exp.  7.  Take  any  piece  of  cut  glass,  such  as  the  stopper  of  a  de- 
canter bottle,  and  hold  it  between  your  eye  and  the  light ;  keep  turning 
it  round,  and  you  will  see  all  the  colors  of  the  rainbow.  A  triangular 
.piece  of  cut  glass,  called  a  glass  prism,  will  answer  the  purpose  best. 
Here  the  rays  of  light  are  decomposed,  by  refraction,  into  their  different 
colored  pencils  of  light. 

Exp.  8.  To  produce  an  artificial  rainboio.  —  When  the  sun  is  shining 
near  the  horizon,  get  a  person  to  project  water  from  a  wet  broom ;  place 
yourself  between  the  sun  and  the  scattered  water,  having  your  face 
towards  the  shower  of  drops,  and  you  will  observe  all  the  colored  tints 
of  the  rainbow.  Here  the  little  drops  of  water  obviously  decompose  the 
light. 

Exp.  9.  (1.)  Observe  that  while  you  blow  a  soap  bubble,  the  varied 
tints  of  the  rainbow  may  be  seen  reflected  from  the  thin  film  of  fluid 
forming  the  bubble.  The  varying  thickness  of  the  film  produces  these 
changes  of  color. 

(2.)  Observe  also  the  colors  exhibited  by  a  drop  of  oil  as  it  spreads 
itself  over  the  surface  of  water. 

(3.)  Take  a  watch  glass  and  a  piece  of  common  window  glass ;  press 
the  two  steadily  together,  and  luminous  rings  will  be  seen  about  their 
point  of  contact. 

These  phenomena  depend  upon  what  has  been  called  the  interference 
of  light. 


144          NATURAL    AND   EXPERIMENTAL   PHILOSOPHY. 

Exp.  10.  (1.)  Hold  a  fine  needle  close  to  one  eye,  the  other  being 
shut ;  and  look  fixedly  at  it,  against  any  light  object  as  a  background ; 
you  will  see  several  needles. 

(2.)  Make  a  straight  cut  in  a  piece  of  card  board ;  look  through  the 
narrow  opening  at  the  candle ;  on  each  side  of  the  real  candle  .you  will 
see  other  candles  marked  with  the  colors  of  the  rainbow. 

(3.)  If  the  light  of  the  sun  be  admitted  into  a  dark  room,  by  a  very 
narrow  chink,  several  luminous  chinks,  separated  by  dark  bands,  will  be 
visible  on  the  opposite  wall. 

(4.)  Suspend  a  black  ball  in  the  sunlight ;  the  round  shadow  of  the 
ball  will  contain  bands  of  light. 

These  phenomena  depend  upon  what  has  been  called  the  diffraction 
of  light. 

REFLECTION    OF   LIGHT   FROM     CONCAVE   AND    CONVEX 
MIRRORS. 

11.  Mirrors  are  divided,  according  to  the  form  of  their 
surfaces,  into  plane,  convex,  and  concave.  The  common 
looking  glass  is  a  plane  mirror. 

The  law  of  the  reflection  of  light,  which  has  been .  ex- 
plained, holds  equally  true  with  respect  to  convex  and  con- 
cave reflectors. 

Let  a  c  b  represent  a  plane  mirror,  fc  g  a  convex  mirror,  and  d  c  e  a 
concave  mirror,  all  touching  each  other  in  the  common  point  c ;  c  I  a 


Fig.  13. 


perpendicular  to  the  plane  a  b ;  k  c  the  direction  of  the  incident  ray,  and 
c  h  the  direction  of  the  reflected  ray ;  then  k  c  I  will  be  the  angle  of  in- 
cidence, and  I  c  h  the  angle  of  reflection,  to  any  of  these  mirrors,  and 
these  angles  will  always  be  equal  to  each  other  —  that  is,  angle  x  will  be 


LIGHT   AND    HEAT. 


145 


equal  to  angle  y,  and,  as  a  necessary  consequence,  angle  w  will  be  equal 
to  angle  z.  It  will  be  observed  that  a  b  forms  a  tangent  to  the  curved 
mirrors  at  the  point  of  contact  c,  and  that  c  I  is  perpendicular  to  the 
curves  at  the  point  ct  being  perpendicular  to  the  tangent  line  a  b. 


14. 


12.     CONCAVE    MIRRORS. 

The  general  effect  of  concave  mirrors  is  to  produce  an 
image  larger  than  the  object  itself. 

Let  s  v  s  (Fig.  14)  represent  a  concave  mirror;  c  its  centre,  that  is,  c 
is  the  centre  of  the  circle  s  v  s ;  r  a  luminous  point,  or  the  flame  of  a 
small  candle;  then* incident  rays  r  s, 
r  s,  falling  upon  the  surface  of  the 
mirror,  will  be  reflected  to  the  same 
point  t,  in  the  directions  s  t,  s  t,  mak- 
ing the  angle  of  incidence  r  s  c  equal 
to  the  angle  of  reflection  c  s  t.  At  the 
point  t,  called  the  focus  of  the  mirror, 
a  small  image  of  the  luminous  object 
will  be  formed,  which  may  be  received 
upon  a  piece  of  thin  white  paper.  The 
line  c  v  produced  is  called  the  axis  of 
the  reflector. 

The  points  r  and  t  are  convertible ;  that  is  to  say,  if  the  flame  of  the 
candle  be  placed  at  t,  then  r  becomes  the  focus,  and  an  enlarged  image 
of  the  candle  will  be  formed  at  this  point. 

When  the  rays  of  light  r  s,  r'  s',  &c., 
(Fig.  15,)  fall  upon  the  concave  mirror 
s  v  s',  parallel  to  the  axis  c  v,  they  are 
reflected  into  a  point  f,  exactly  midway 
between  the  reflector  and  its  centre  c. 
The  point /is  called  the  principal  focus t 
and  its  distance  fv  from  the  speculum  or 
reflector  is  called  the  principal  focal  dis- 
tance. 

In  Fig.  14,  f  is  the  principal  focus, 
making  /  v  =f  c ;  t  is  a  focus  to  the 

rays  proceeding  from  a  luminous  point  at  r ;  in  this  case  the  incident 
rays  are  not  parallel  to  the  axis  ;  hence,  by  way  of  distinction,  the  focus 
of  parallel  rays  is  called  the  principal  focus. 

When  the  rays  of  light  come  from  a  point  r  beyond  the  centre  c, 
as  in  Fig.   li,  the  reflected  rays  will  all  converge   to  the    focus    t 
13 


\ 


146 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


between  the  principal  focus  f  and  the  centre  c 
of  the  speculum ;  on  the  contrary,  if  the  rays 
of  light  come  from  a  point  r  (see  Fig.  16)  be- 
tween the  mirror  and  the  principal  focus,  the 
reflected  rays  s  g,  s  q',  &c.,  will  all  diverge  from 
the  axis  V  c.  If  the  luminous  body  be  placed  in 
the  principal  focus  /,  (see  Fig.  15,)  then  the  re- 
flected rays  s  rt  s  r't  &c.,  will  all  be  parallel  to 
one  another. 

The  image  of  the  object  is  inverted.  Let  D  E 
be  the  concave  mirror,  (see  Fig.  17  ;)  C  its  centre  ;  A  B  a  distant  object 
placed  before  it ;  and  a  b  its  inverted  image,  reflected  from  the  mirror. 
Here  the  incident  rays  A  D,  proceeding  from  the  f>oint  of  the  arrow, 


Fig.   16. 


Fig.  17. 

are  reflected  in  the  direction  D  a,  making  the  angle  of  reflection  C  D  a 
equal  to  the  angle  of  incidence  ADC;  and  the  incident  rays  B  E,  pro- 
ceeding from  the  foot  of  the  arrow,  are  reflected  in  the  direction  E  b, 
making  the  angle  of  reflection  C  E  b  equal  to  the  angle  of  incidence 
BEG;  and  so  on  to  other  points  ;  thereby  producing  the  little  inverted 
image  a  b  of  the  arrow  in  the  focus  of  the  reflector. 

If  a  small  object  be  placed  at  the  focus  a  b,  then  an  enlarged  image 
will  be  formed  at  A  B. 

Exp.  1.  Place  a  candle  at  a  distance,  opposite  to  a  concave  mirror, 
(see  Fig.  17  ;)  hold  a  thin  piece  of  white  paper  near  the  surface  of  the 
mirror ;  a  small,  bright,  inverted  image  of  the  candle  will  be  thrown  upon 
the  paper ;  move  the  paper  backwards  or  forwards  until  you  have  got 
the  exact  focus.* 

Exp.  2.  Reverse  the  last  experiment,  that  is  to  say,  place  the  candle 
in  the  focus,  and  then  an  enlarged  image  will  be  thrown  upon  the  paper 
placed  at  A  B. 

Exp.  3.  Hold  the  concave  mirror  facing  the  sun ;  hold  a  piece  of 
paper  in  the  focus,  and  an  intensely  bright  image  will  be  formed,  which 


A  common  tin  reflector  answers  very  well  for  making  these  experiments. 


LIGHT    AND    HEAT. 


147 


will  almost  instantly  ignite  the  paper.  This  forms  a  burning  mirror. 
In  this  case  the  luminous  image  lies  in  the  principal  focus  of  the  re- 
flector. 

Exp.  4.  Place  the  candle  in  the  principal  focus  of  the  reflector  ;  the 
rays  of  light  will  all  be  reflected  parallel  to  the  axis,  and  will  illuminate 
the  wall  of  the  room  upon  which  they  are  thrown.  Bring  the  candle 
still  nearer  to  the  reflector,  and  the  reflected  rays  will  all  diverge  from 
the  axis,  and  will  illuminate  a  greater  extent  of  surface. 

Exp.  5.  Place  the  object  N  S  between  the  mirror  and  principal  focus 


Fig.  18. 

/,  as  shown  in  Fig.  18  ;  an  enlarged  image  of  the  object  will  be  seen,  in 
its  erect  position,  behind  the  mirror. 

Exp.  6.  The  magic  mirror.  —  Place  a 
small  object  s  n,  concealed  from  the  view 
of  the  observer,  between  the  centre  c  and 
the  principal  focus  f;  receive  the  enlarged 
image  N  S  through  an  opening  cut  in  a 
board  ;  look  in  the  direction  of  this  opening, 
and  you  will  see  the  image  suspended,  as  it 
were,  in  the  air. 

This  effect  is  easily  explained  on  the  prin- 
ciples which  have  been  already  expounded. 


Fig.  19. 


13.     CONVEX   MIRRORS. 

The  principal  focus  of  a  convex  mirror  lies  as  far  behind 
the  reflecting  surface  as  in  concave  mirrors  it  lies  before  it. 
The  focus  in  this  case  is  called  the  virtual  focus,  because  it  is 
only  an  imaginary  point,  towards  which  the  rays  of  reflection 
appear  to  be  directed. 

Let  N  S  be  an  object  placed  before  a  convex  mirror,  (see  Fig.  20 ;) 
c  the  geometrical  centre  of  the  mirror ;  /the  principal  focus ;  N  A  an 


148 


NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 


incidental  ray ;  c  A  D  a  straight  line  drawn 
from  the  centre  c ;  A  B  the  course  of  the 
reflected  ray,  making  the  angle  of  reflec- 
tion DAB  equal  to  the  angle  of  incidence 
NAD;  then  a  small  direct  image  n  s  will 
be  formed  in  the  focus. 


The  general  effect  of  convex  mirrors  is  to  produce  an 
image  smaller  than  the  object  itself. 

The  phenomena  of  reflected  images  is  more  fully  exhibited  in  Pigs. 
21  and  22. 


Fig.  21, 


Fig.  22. 


The  incident  rays  M  D,  M  F,  are  seen  in  the  directions  D  m,  F  m  ; 
and  N  G,  N  H,  are  seen  in  the  directions  G  n,  H  n  ;  where  the  distance 
A  m  and  F  n  behind  the  mirrors  have  a  certain  correspondence  with  the 
respective  distances  of  the  points  M  and  N  in  front  of  the  mirrors. 


THE   REFRACTION    OF  LIGHT. 

14.  The  following  remarkable  law  obtains  in  relation  to  the 
refraction  of  light :  When  a  ray  of  light  passes  from  one 
transparent  medium  to  another,  the  sine  of  the  angle  of  inci- 
dence has  always  a  constant  ratio  to  the  sine  of  the  angle  of 
refraction. 

Suppose  m  n  be  the  surface  of  water,  (see  Fig.  23  ;)  a  b  the  incident 
ray,  making  with  the  perpendicular  h  b  d  the  angle  of  incidence  x  or 
ab  d;  5  c  the  refracted  ray  bent  from  the  right  line  a  bf,  and  forming 
the  angle  of  refraction  y,  or  c  b  h.  With  b  as  a  centre,  describe  the  circle 
a  d  c;  and  on  h  d  let  fall  the  perpendiculars  a  g  and  c  h  ;  then  a  g  is 


LIGHT    AND    HEAT. 


149 


called  the  sine  of  the  angle  of  inci- 
dence, and  c  h  is  called  the  sine  erf 
the  angle  of  refraction,  and  those  two 
lines  have  always  a  constant  ratio  to 
each  other,  viz.,  as  4  to  3,  or  as  1.336 
to  1,  whatever  (within  certain  re- 
strictions) may  be  the  angle  at  which 
the  ray  a  b  meets  the  surface  of  the 
fluid.  The  number  1.336  is  called 
the  index  of  refraction  for  water. 
When  the  medium  is  flint  glass,  the 
sine  of  the  angle  of  incidence  a  g  is 
to  the  sine  of  the  angle  of  refraction 
c  h  as  3  to  2,  or  as  1.5  is  to  1  nearly. 
The  number  1.5  is  called  the  index  of  refraction  for  flint  glass.  Glass, 
therefore,  has  a  higher  refractive  power  than  water.  Generally  speaking, 
the  denser  the  medium  the  higher  is  its  refractive  powers. 

When  a  ray  of  light  passes  from  a  rare  to  a  dense  medium, 
as,  for  example,  from  air  to  water,  the  refracted  ray  is  bent 
towards  the  perpendicular ;  so,  conversely,  when  the  ray 
passes  from  a  dense  to  a  rare  medium,  the  refracted  ray  is 
bent  from  the  perpendicular. 

Newton  accounted  for  the  refraction  of  light  on  the  supposition  that 
media  of  different  compositions  exert  an  attractive  power  on  the  rays  of 
light  when  they  approach  their  respective  boundaries :  thus  the  fluid 
m  n  (see  Fig.  23)  is  supposed  to  exert  an  attractive  influence  upon  the 
incident  ray  a  b,  when  it  approaches  the  surface  m  n,  and  thereby  bends 
the  course  of  the  ray  downwards. 

Passage  of  a  Ray  through  a  Plate  of  Glass. 

15.  The  ray  will  be  bent  downwards  upon  enter- 
ing the  plate,  but  it  will  be  bent  as  much  in  the  con- 
trary direction  upon  passing  out  of  the  plate,  as 
shown  in  Fig.  24,  so  that  the  course  of  the  ray  after 
refraction  will  be  parallel  to  the  direction  which  it 
had  before  refraction. 


Fig.  24 


Passage  of  a  refracted  Ray  through  a  Prism. 

16.   Let  C  (Fig.  25)  be  a  prism  of  glass  ;  a  r  the  incident  ray  falling 
perpendicularly  on  one  face  of  the  prism  ;  then  the  ray  will  undergo  no 
13* 


150 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


25. 


refraction  on  entering  the  prism,  for  it  will  pur- 
sue the  straight  course  arc;  but  upon  leaving 
the  oblique  face  of  the  prism,  the  ray  will  be  bent 
towards  the  surface  of  the  glass,  or,  what  is  the 
same  thing,  it  will  be  bent  from  the  perpendic- 
ular c  m,  and  will  move  on  in  the  direction  c  E. 
If  the  incident  rays  had  been  inclined  to  the 
plane  of  the  first  face,  they  would  have  unde 
gone  two  different  refractions. 

Tlie  Multiplying   Glass. 

17.  A  D  C  B  represents  a  section  of  a  piece  of  cut  glass  having  three 
faces,  A  D,  D  C,  and  C  B,  inclined  to  one  another, 
as  shown  in  Fig.  26  ;  d  an  object  placed  in  front 
of  the  face  A  B  ;  then  an  eye  at  a  will  see  three 
distinct  images  of  the  object.  The  rays  d  b  un- 
dergo refraction  in  passing  from  the  face  A  D,  and 
d  b  in  passing  from  B  C,  and  so  on  to  the  other 
faces.  The  number  of  images  seen  always  corre- 
sponds to  the  number  of  inclined  faces  in  the  mul- 
tiplying glass. 

Refraction  in  Lenses,  or  Glasses  with  curved  Faces. 
18.  There  are  six  different  forms  of  simple  lenses. 

No.  I.,  Fig.  27,  represents  a  double  convex  lens ;  No.  II.  a  plano- 
convex lens ;  No.  III.  a  meniscus  lens,  like  a  watch  glass ;  No.  IV.  a 
double  concave  lens ;  No.  V.  a  plano-concave  lens ;  and  No.  VI.  a  con- 
cavo-convex lens. 

m* 


Fig.  27. 


Fig.  28. 


LIGHT   AND    HEAT. 


151 


Definitions  relative  to  lenses.  —  The  line  p  q  (see  Fig.  28)  is  called 
the  diameter ;  c  the  geometric  centre ;  c  n  the  axis ;  m  the  optical 
centre ;  I  m  o  a  principal  when  it  passes  through  the  optical  centre. 

Radii,  c  n,  c  s,  &c.,  being  at  right  angles  to  the  curved  surface,  con- 
stitute the  perpendiculars  from  which  the  angles  of  incidence  and 
refraction  are  estimated. 


FOCAL    DISTANCES    OF    LENSES. 

19.  Double  convex  lenses.  —  When  the  incident  rays  are  parallel,  the 
distance  of  the  focus  f  is  equal  to  the  radius  of  the  spherical  surface,  as 
shown  in  Fig.  29.  Here/  is  called  the  principal  focus. 

"When  the  incident  rays  are  divergent,  as  in  Fig.  30,  the  focus  r  lies 
beyond  the  principal  focus/. 


> — -f 

Fig.  29.  Fig.  30. 

When  the  incident  rays  are  convergent,  as  in  Fig.  31,  the  focus  r  lies 
within  the  principal  focus. 

20.  Plano-convex  lenses.  —  When  the  incident  rays  are  parallel,  the 
distance  of  the  focus  r  is  equal  to  the  diameter'of  the  spherical  surface, 
as  shown  in  Fig.  32. 


Fig.  31.  Fig.  32. 

21.  In  the  double  convex  lens  L  L,  Fig.  33,  O  and  O'  are  the  centres 
of  the  surfaces  of  the  lens,  and  also  the  principal  foci  ;  R  the  radiating 


Fig.  33. 


152 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


point ;  F  the  focus  ;  and  when  R  approaches  to  P,  the  focus  F  recedes 
to  P' ;  and  when  E,  comes  to  O',  the  focus  is  infinitely  distant. 

22.  Double  concave  lenses.  —  Divergent  incident  rays  diverge  still  more 
after  refraction,  and  seem  to  proceed  from 
a  point  nearer  to  the  lens  than  that  from 
which  they  actually  proceed,  as  shown 
in  Fig.  34,  where  the  rays  proceeding 
from/'  are  refracted  towards  r,  and  ap- 
pear as  if  they  had  emanated  from  r1. 
And  so  on  to  other  cases. 

The  foci  of  lenses  may  be  readily  found 
by  the  methods  of  trial  explained  at 
page  146,  &c. 


Fig.  34. 


IMAGES    OF    OBJECTS    FORMED    BY    LENSES. 

23.  Convex  lenses.  —  If  the  object  N  S  lie  beyond  the  principal  focus, 
as  shown  in  Fig.  35,  the  image  n  s  will  be  inverted.  If  the  object  be 
very  remote,  as  in  the  case  of  the  sun,  then  the  image  will  be  formed  in 
the  principal  focus  f;  in  this  case  the  image  will  be  very  small.  If  the 


Fig.  35. 

distance  of  the  object  be  equal  to  twice  the  principal  focal  distance,  the 
image  will  be  at  the  same  distance  on  the  other  side  of  the  lens,  and  of 
the  same  size  as  the  object. 

If  the  distance  of  the  object  be  still  further  diminished,  yet  not  within 


Fig.  36. 

the  principal  focal  distance,  the  image  will  recede  from  the  lens,  and  its 
dimensions  will  be  increased  accordingly,  a^s  shown  in  Fig.  36. 

If  the  object  N  S  be  brought  within  the  principal  focal  distance,  as 
shown  in  Fig.  37,  an  eye  at  the  focus/ will  see  an  enlarged  image  of  the 


LIGHT    AND    HEAT. 


153 


object  at  n  s.     In  this  case  the  refracted  rays  do  not  cross  each  other,  and 
hence  the  image  is  seen  erect. 


Fig.  37. 

Convex  lenses  are  called  MAGNIFYING  GLASSES,  because 
they  thus  increase  the  apparent  size  of  objects  viewed  through 
them. 

24.  Concave  lenses.  —  These  lenses  diminish  the  apparent 
size  of  objects ;  hence  they  are  called  DIMINISHING  GLASSES, 

An  object  N  S,  (see  Fig.  38,) 
viewed  from  the  point  f,  will  pre- 
sent a  small  image  n  s  in  the  vir- 
tual focus. 

All  the  phenomena  relative  to 
convex  and  concave  lenses  are  pre- 
cisely analogous  to  those  produced 
by  concave  and  convex  mirrors. 


Fig.  38. 


25.  Distortion  of  images  produced  by  spherical  aberra- 
tion. —  The  images  of  objects  produced  by  spherical  lenses 
and  mirrors  are  only  true  for  the  rays  which  lie  near  to  the 
axes.  The  rays  which  fall  at  a  distance  from  the  axes  pro- 
duce distortions  in  the  images,  which  have  been  called  spheri- 
cal aberrations.  In  order,  therefore,  to  produce  a  tolerably 
correct  image,  it  is  necessary  that  the  extreme  rays  falling 
upon  the  lenses,  or  upon  the  mirrors,  as  the  case  may  be, 
should  be  excluded  from  the  other  rays  forming  the  picture. 

There  are  other  devices  for  effecting  this  purpose,  which  would  be 
foreign  to  the  object  of  this  work  to  explain  very  minutely. 

Descartes  discovered  that  a  concave-convex  lens,  A  L  a  L,  (Fig.  39,) 
having  for  its  convex  surface  a  portion  of  an  ellipse,  could  be  made  so  as 
to  correct  any  spherical  aberration  ;  and  Sir  J.  Herschel  discovered  that 
the  same  might  be  effected  by  two  lenses,  A  B  and  C  D,  (Fig.  40.) 


154          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  39 


26.  Caustic  curves  formed  by  reflection.  —  The  rays  of  light  reflected 
from  the  different  points  of  a  concave  reflector  MEN,  (Fig.  41,)  cross 
one  another  at  particular  points,  and  thus  a  luminous  curve  of  reflected 


light,  known  by  the  name  of  the  caustic  curve,  is  formed.  R  1,  R  2, 
K  3,  &c.,  are  the  incident  rays  proceeding  from  the  luminous  point  R, 
and  1  1,  2  2,  3  3,  &c.,  are  their  respective  reflected  rays ;  the  luminous 
intersections  form  the  caustic  curve  M  F  N. 

To  observe  this  curve,  place  a  lighted  candle  at  a  little  distance  from  a 
basin  about  one  half  full  of  milk ;  then  a  luminous  curve  will  be  seen 
upon  the  surface  of  the  milk. 


OPTICAL    INSTRUMENTS. 

THE    HUMAN    KYE. 


27.  The  eye  is  a  lens  of  the  most  delicate  and  elaborate 
construction.     The  eye  is  so  constructed  that  it  forms  images 


LIGHT  AND    HEAT.  155 

of  external  objects  upon  a  thin  screen  of  nerves  communicat- 
ing with  the  brain,  and  thus  the  sensation  of  vision  is  pro- 
duced. There  is  nothing  in  nature  which  more  fully  demon- 
strates the  existence  of  a  great  and  beneficent  Creator  than 
the  adaptation  of  the  human  eye  to  the  purposes  for  w.hich  it 
is  designed  to  serve.  Let  us  look  more  minutely  into  the 
construction  of  this  wonderful  organ. 

The  eye  is  nearly  spherical  in  figure  ;  it  consists  of  several 
membranes  or  coats,  the  anterior  or  front  portions  of  which 
are  transparent,  so  as  to  admit  the  rays  of  light  proceeding 
from  external  objects  into  the  interior  of  the  eye.  These 
coats  enclose  two  colorless  fluids  or  humors,  separated  from 
each  other  by  membranes ;  the  anterior  portion  being  called 
the  aqueous  humor,  and  the  posterior  portion  the  vitreous 
humor.  In  the  centre  of  this  partition  is  a  circular  aperture, 
or  hole,  for  the  .admission  of  light,  called  the  pupil  of  the  eye, 
behind  which  is  a  double  convex  lens,  called  the  crystalline 
lens.  Opposite  to  this  lens  is  the  optic  nerve,  which  extends 
itself  over  the  inner  surface  of  the  eye.  The  eye  is  sur- 
rounded by  bones,  and  is  moved  by  various  muscles.  The 
optic  nerves  of  both  eyes  unite  in  a  common  nervous  cord 
which  communicates  with  the  brain. 

Fig.  43  represents  a  front  view  of  the  eye ;  and  Fig.  42  a  sectional 
view  of  it.  The  same  letters  of  reference  are  used  in  both  figures. 


Fig.  42.  Fig.  43. 


a  k  d  c,  the  outermost  membrane,  is  called  the  Sclerotic  coat :  it  forms 
the  white  of  the  eye  ;  a  b  c,  the  projecting  transparent  part,  is  called  the 
Cornea ;  e  is  the  Crystalline  lens  suspended  between  the  Ciliary  Processes 
g  h,  which  divide  the  eye  into  two  chambers,  /  and  g  m  n  h  ;  the  smaller 
and  anterior  portion  I  is  filled  with  the  Aqueous  humor,  and  the  larger 


156          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

and  posterior  portion  with  the  Vitreotts  humor  ;  the  former  humor  is  like 
water,  and  the  latter  somewhat  like  a  jelly,  but  both  are  colorless  and 
highly  transparent,  and  have  about  the  same  refractive  powers  as  water, 
which  is  also  the  case  with  respect  to  the  crystalline  humor,  g  m  n  h  is 
the  Choroid  coat,  lining  the  whole  of  the  interior  surface  of  the  sclerotic 
coat,  in  the  form  of  a  black  slimy  pigment  or  paint,  to  prevent  any  re- 
flection of  light  taking  place  within  the  eye.  Between  the  crystalline 
lens  and  the  cornea  is  the  Iris  kale,  which  gives  the  peculiar  color  to 
the  ring  of  the  eye,  in  the  middle  of  which  is  the  Pupil  I,  which  has  the 
power  of  expanding  and  contracting  to  suit  the  intensity  of  the  light. 
f  is  the  Optic  Nerve,  which  passes  through  the  sclerotica  and  spreads 
itself  over  this  coat  in  a  reticulated  form  m  n,  or  in  the  form  of  network, 
and  is  called  the  Retina.  The  nerves  of  the  two  eyes,  as  we  have  already 
observed,  unite  in  a  common  nervous  cord  which  communicates  with  the 
brain. 

Now,  when  rays  of  light  from  any  luminous  object  fall  upon  the  eye, 
they  pass  through  the  pupil,  and  then  become  refracted  by  the  crystal- 
line lens  in  the  same  manner  as  by  any  other  double  convex  lens,  and 
then  converge  to  a  focus  at  the  retina,  where  a  small  inverted  image  of 
the  object  is  formed,  which,  acting  on  the  fine  network  of  nerves,  pro- 
duces the  sensation  of  vision.  It  must  be  observed  that  the  aqueous  and 
vitreous  humors  also  influence  the  refraction  of  the- light. 

When  the  lenses  of  the  eye  are  too  round,  or,  it  may  be, 
too  dense,  the  rays  of  light  are  brought  to  a  focus  before  they 
reach  the  retina :  this  takes  place  with  short-sighted  people ; 
on  the  contrary,  when  the  lenses  are  too  flat,  or  too  thin,  the 
focus  of  the  rays  lies  beyond  the  "retina :  this  takes  place  with 
aged  people,  who  are  said  to  be  long-sighted.  To  correct  the 
focus  of  vision,  short-sighted  persons  use  concave  glasses,  and, 
on  the  contrary,  long-sighted  persons  use  convex  glasses. 

An  object  appears  less  and  less  as  its  distance  from  us  is  increased. 
Thus  the  arrow  at  A  B  (Fig.  44)  will  appear  larger  to  the  eye  E  of  a 
person  than  it  would  do  at  D  C.  If  the  apparent  lengths  of  the  arrow 
in  these  two  positions  be  measured  by  means  of  a  pencil  or  little  rod,  we 
shall  find  that  the  arrow  at  A  B  will  appear  of  the  size  a  b  on  the  pencil, 
whereas  the  arrow  at  D  C  will  appear  only  of  the  size  d  c  on  the  pencil. 

In  judging  of  the  actual  size  of  an  object,  we  always  take 
into  account  the  distance  at  which  it  is  seen. 

Thus,  although  a  boy  near  at  hand  may  appear  to  us  larger  than  a 


LIGHT   AND    HEAT. 


157 


Fig.  44. 

man  at  a  distance,  yet  we  always  form  a  correct  idea  of  their  relative 
dimensions  by  making  an  allowance  for  the  effect  of  distance. 

We  form  a  judgment  of  the  distance  of  an  object  by  the 
number  and  size  of  the  intervening  objects,  and  by  the  dis- 
tinctness or  indistinctness  of  its  outline. 

The  spire  of  a  church,  as  it  appears  in  the  far  horizon  piercing  the  sky, 
may  be  very  lofty,  or  it  may  be  scarcely  higher  than  an  ordinary  build- 
ing ;  but  there  are  men  and  carriages,  fields  and  cattle,  forests  and  houses, 
hills  and  valleys,  between  us  and  that  spire,  and,  besides,  the  windows  in 
its  tower  are  so  indistinct  that  they  can  scarcely  be  distinguished ;  from 
all  this  we  conclude  that  the  spire  is  a  great  distance  off,  and  that  it  is 
very  lofty.  A  man  seen  through  a  fog  sometimes  appears  to  us  like  a 
giant ;  how  is  this  ?  The  fog,  while  it  throws,  as  it  were,  a  veil  over 
the  intervening  objects,  causes  the  object  to  appear  indistinct,  and  there- 
by gives  us  a  false  impression  with  respect  to  its  actual  distance  ;  that  is 
to  say,  the  fog  causes  us  to  believe  that  the  man  is  at  a  greater  distance 
from  us  than  he  really  is,  and  thus  we  are  led  to  assign  to  him  an  unu- 
sual magnitude.  We  make  the  same  allowance  for  distance,  &c.,  with 
respect  to  the  objects  represented  in  a  picture,  that  we  do  when  looking 
at  the  actual  objects. 

The  angle  formed  by  the  rays  of  light  passing  from  the  top 
and  bottom  of  an  object  to  the  eye  is  called  the  visual  angle 
or  the  optic  angle. 

Thus  the  visual  angle  of  the  arrow  at  A  B  (see  Fig.  44)  is  the  angle 
A  E  33 ;  whereas  the  visual  angle  of  the  arrow  at  D  C  is  the  angle 
DEC,  where  it  will  be  observed  that  the  visual  angle  formed  by  an  ob- 
ject becomes  less  and  less  as  the  object  recedes  from  the  eye ;  or,  what 
amounts  to  the  same  thing,  the  apparent  magnitude  of  an  object  is  in 
proportion  to  its  visual  angle. 
14 


158          NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

Ill  like  manner,  the  traveller  at  b<  d'  will  have  the  same  apparent 


magnitude  to  the  eye  at  a  as  the  distant  cross  d,  because  the  visual  an- 
gles b'  a  d'  and  bad  are  equal  to  each  other. 

In  order  to  understand  the  way  in  which  the  eye  receives  impressions 
of  objects,  let  us  suppose  that,  in  Fig.  46,  K  represents  a  section  of  the 
human  eye,  P  the  pupil  in  front,  E  the  crystalline  lens,  in  which  all  the 
rays  are-  refracted  and  cross  each  other,  k  q  the  concave  surface  of  the 
back  of  the  eye,  called  the  retina,  on  which  the  image  of  the  object  is 
projected ;  moreover,  let  us  suppose  that  the  eye  of  the  person  is  looking 
at  the  cross  A  B  C  D,  and  that  Q  H  O  N  represents  a  picture  frame  in 
which  a  pane  of  glass  is  inserted,  having  its  surface  coated  with  gum 
arabic  so  that  chalk  lines  may  be  traced  upon  it,  giving  the  picture  cb  da 
of  the  cross :  then  rays  of  light  will  proceed  from  every  part  of  the  cross 
C  B  to  the  eye,  or,  what  is  the  same  thing,  from  every  part  of  the  pic- 
ture c  b  to  the  eye,  and  will  form  the  inverted  image  m  p  of  the  cross 
upon  the  retina ;  thus  it  will  be  understood  that  the  object  and  its  pic- 
ture would  form  the  same  image  upon  the  retina,  for  the  point  b  inter- 


Fig.  46. 

cepts  the  view  of  B,  c  that  of  C,  a  that  of  A,  and  so  on.  Now,  if  we 
move  the  cross  B  C  to  F  G,  the  picture  f  g  on  the  glass,  as  well  as  the 
image  k  q  upon  the  retina,  would  be  much  larger  ;  thus  it  appears  that 
the  image  on  the  retina  is  larger  or  smaller  as  the  object  advances  to  or 
recedes  from  the  eye  of  the  spectator. 

When  an  object  is  brought  too  near  the  eye,  the  angle  of  vision  and 


LIGHT    AND    HEAT.  159 

the  image  of  the  object  become  so  enlarged  that  the  image  is  thrown 
beyond  the  retina,  which  occasions  us  to  see  the  object  indistinctly. 
Persons  of  ordinary  vision  cannot  see  objects  distinctly  when  they  are 
within  the  distance  of  six  or  eight  inches  from  the  eye. 

THE   MICROSCOPE. 

28.  The  microscope  magnifies  the  images  of  minute  ob- 
jects, and  enables  us  to  see  them  with  greater  distinctness. 
This  is  effected  by  enlarging  the  visual  angle  ;  for,  as  we  have 
shown,  every  object  appears  larger  according  as  we  increase 
this  angle. 

The  Single  Microscope. 

29.  The  single  microscope  con- 
sists of  a  single  convex  lens  m, 
with  a  very  short  focal  distance. 
An  eye  at  a  (see  Fig.  47)  would 
see  the  arrow  b  c  under  the  visual 
angle  b  a  c ;  but  when  the  lens  m 
is  interposed,  it  is  seen  under  the 

visual  angle  B  a  C,  and  hence  it  pig.  47. 

appears  much  enlarged,  as  shown 

in  the  image  B  C.    The  principles  of  refraction  upon  which  this  depends 

have  already  been  explained. 

In  order  to  see  the  image  distinctly,  it  is  of  course  requisite  that  the 
object  should  be  placed  in  the  focus  of  the  lens. 

Concave  mirrors  may  be  also  used  as  microscopes.     (See  Fig.  19.) 

Tlie  Compound  Microscope. 

30.  The  compound  microscope  consists  of  two  or  more  convex  lenses, 
or  of  a  combination  of  lenses  and  concave  mirrors. 

Fig.  48  represents  a  compound  microscope  consisting  of  two  convex 
lenses  B  and  C.  The  first  lens  B  is  called  the  object  glass,  and  the  sec- 
ond C  the  eye  glass,  a  b  is  the  object ;  a1  b1  the  inverted  magnified 
image  formed  by  the  lens  B  ;  A  the  eye  of  the  observer  ;  az  b*  the  image 
magnified  again  by  the  lens  C,  and  seen  under  the  enlarged  visual  angle 
a?  A  b2.  Now,  if  we  suppose  the  lens  B  to  have  a  magnifying  power  of 
25,  —  that  is,  if  the  image  o>  b1  equals  25  times  a  b,  and  the  lens  C  to 
have  a  magnifying  power  of  4,  —  then  the  total  magnifying  power  of 
the  microscope  will  be  4  times  25,  or  100  —  that  is  to  say,  the  image  of 


160 


NATURAL   AND   EXPERIMENTAL   PHILOSOPHY. 


the  object  will  appear  100  times  the  size  of  the  object,  and  the  visual 


Fig.  48. 

angle  a2  A  62  will  be  100  times  the  visual  angle  which  the  object  itself 
would  form  with  the  eye  at  A. 

The  microscope  enables  us  to  see  the  structure  of  various  minute  ob- 
jects. The  drawings  shown  in  Fig.  49  represent  the  appearance  of  some 
minute  objects  when  seen  through  a  tolerably  good  microscope.  A  rep- 
resents the  wing  of  a  small  insect  called  menelaus  ;  B  and  C  the  hair  of 
the  bat ;  and  D  and  E  the  hair  of  the  mouse. 


Fig.  49. 

Sometimes  mirrors  are  placed  beyond  the  objects,  to  throw  a  greater 
amount  of  light  upon  them. 

THE   TELESCOPE. 

31.  Telescopes  are  used  to  magnify  the  images  of  distant 
objects  ;  and  this  is  done  in  the  same  manner  as  in  the  micro- 
scope, viz.,  by  enlarging  the  visual  angle  at  which  they  are 
seen. 

There  are  two  kinds  of  telescopes  used  —  refracting  tele- 
scopes and  reflecting  telescopes. 


LIGHT    AND    HEAT. 


161 


Refracting  Telescopes. 

32.  The  astronomical  telescope  is  represented  in  Fig.  50.  It  consists 
of  two  convex  lenses,  C  and  B,  of  unequal  size  and  focal  length.  The 
eye  glass  B  has  a  much  greater  magnifying  power  than  the  object  glass 


Fig.  50. 

C,  which  is  just  the  reverse  of  what  is  observed  in 
the  construction  of  the  compound  microscope  rep- 
resented in  Fig.  48;  the  distance  of  the  lenses 
from  each  other  is  usually  equal  to  the  sum  of 
their  focal  lengths ;  the  eye  glass  B  is  fixed  in  a 
sliding  tube  for  the  purpose  of  adjusting  the  focal 
distance  between  the  two  lenses  to  suit  the  vary- 
ing distance  of  objects,  a  b  is  the  distant  object ; 
a'  b'  its  image  formed  by  the  lens  C ;  B  the  eye 
glass  which  magnifies  this  image,  so  that  it  is  seen 
by  the  eye  A  magnified  at  a2  b2. 

The  night  glass  is  similar  in  its  construction  to 
the  astronomical  telescope. 

33.  The  terrestrial  telescope  is   represented  in 
Fig.  51  ;  it  usually  consists  of  four  convex  lenses, 
L,  O1,  O2,  and  O3  ;  it  may,  therefore,  be  regarded 
as  a  double  astronomical  telescope.     These  instru- 
ments show  objects  in  their  natural  position.     The 
lens  L  has  a  great  focal  length ;  O1,  O2,  and  O3 
are  three  double  convex  eye  glasses,  having  short 
equal  focal  distances  set  in  the  same  sliding  tube, 
so  that  the  posterior  focus  of  one  lens  may  exactly 
coincide  with  the  anterior  focus  of  the  next. 

This  sliding  tube  enables  the  observer  to  adjust 
for  the  focus  of  the  field  glass  L. 

34.  The  Galilean  telescope  is  represented  in  Fig. 
52  ;  it  consists  of  a  convex  object  glass  L  and  a 
plano-convex  eye  glass  O.     The  inverted  image 
a'  b'  which  would  be  formed  but  for  the  lens  O, 

14* 


Fig.  51. 


162          NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 


Fig.  52. 

•which  by  its  refraction  causes  the  rays  to  diverge  from  one  another,  and 
thereby  forms  the  erect  image  a2  62,  which,  of  course,  is  seen  by  the  eye 
at  an  enlarged  visual  angle.  This  instrument  is  now  chiefly  used  as  an 
opera  glass. 

-Achromatic  Lenses. 

35.  The  instruments  just  described  have  two  great  defects  :  (1.)  The 
defect  arising  from  spherical  aberration  ;  (2.)  The  defect  arising  from  the 
colored  light  produced  by  the  prismatic  decomposition  of  the  light.  (See 
page  153.)  In  order  to  remedy  these  defects,  Dolland  invented  what  are 
called  achromatic  lenses. 

The  achromatic  lens  represented  in  Fig.  53  con-  j  "rD  Re- 
sists of  a  plano-concave  flint  glass  fitted  on  one  L<-^ /r^)wf 
face  of  a  double  convex  crown  glass.  ^<;;;<^__^_ 

Light,  upon  passing  through  a  glass  lens,  is  dis-  Fig.  53. 

fersed  —  that  is,  the  light  is  separated  into  differ- 
ent  colored  rays.  Now,  crown  and  flint  glass  differ  considerably  in  their 
dispersive  powers,  and  at  the  same  time  differ  very  little  in  their  refrac- 
tive powers ;  hence  the  contrivance  of  the  achromatic  lens  simply  con- 
sists in  making  the  dispersive  power  of  the  one  glass  exactly  to  counter- 
act the  dispersive  power  of  the  other,  and  thereby  to  destroy  the  effect 
of  what  is  called  chromatic  aberration. 

Fig.  54  represents  the  achromatic  eye  piece  now  in  general  use  in  all 
good  achromatic  telescopes  for  land  objects.  It  consists  of  four  lenses, 


Fig.  54. 


A,  C,  D,  and  B.     A  is  very  nearly  a  plano-convex  lens ;  C  a  meniscus  ; 
D  a  nearly  plano-convex  lens  ;  and  B  a  double  convex  lens.* 

*  For  the  radii  and  distances  of  these  lenses,  the  reader  may  consult 
Brewster's  Optics. 


LIGHT    AND    HEAT. 


163 


Craig's  Achromatic   Telescope. 

36.  This  is  the  largest  instrument  of  the  kind  that  was  ever  executed. 
The  achromatic  object  glass  is  24  inches  in  diameter,  and  has  a  focal  dis- 
tance of  76  feet.  The  manner  of  fitting  up  this  magnificent  instrument 
is  shown  in  Fig.  55.  The  telescope  is  suspended  on  the  side  of  a  strong 


Fig.  55. 

tower,  64  feet  high  and  75  feet  in  diameter.  The  length  of  the  main 
portion  of  the  tube  is  76  feet ;  and,  with  the  eye  piece  at  the  smaller 
end  and  the  dew  cap  at  the  greater  end,  the  total  length  of  the  telescope 
is  85  feet.  The  lower  extremity  of  the  tube  rests  upon  a  wooden  frame 
standing  on  wheels,  which  run  on  a  circular  railway  going  round  the 
tower  at  the  distance  of  about  15  feet  from  it,  so  that  the  instrument 
admits  of  being  readily  directed  to  any  portion  of  the  heavens. 


164         NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


REFLECTING   TELESCOPES. 

37.   The  Gregorian  telescope,  represented  in  Fig.  56,  consists  of  two 
concave  mirrors  or  specula  s  and  S  S,  with  their  concave  surfaces  facing 


Fig.  56. 

each  other,  and  a  double  convex  eye  glass  o,  the  whole  being  fitted  in  a 
metallic  tube,  a  b  is  the  distant  object ;  a1  6l  its  inverted  image,  formed 
by  the  large  concave  mirror  or  speculum  S  S ;  this  image  is  again  re- 
flected by  the  small  mirror  s,  and  thus  forms  the  erect  image  a2  Z>2,  which 
is  magnified  by  the  lens  o  into  the  image  a3  b3,  when  observed  by  an 
eye  at  A. 

38.  The  Newtonian  telescope,  represented  in  Fig.   57,  consists  of  a 
large  concave  mirror  S  and  a  small  plane  mirror  p  placed  obliquely  to 


Fig.  57. 

the  direction  of  the  axis  of  the  tube,  (at  an  angle  of  45°,)  and  a  magni- 
fying lens  o  placed  in  the  side  of  the  tube. 

39.  Herschel's  telescope,  represented  in  Fig.  58,  has  only  one  concave 


Fig.  58. 


LIGHT    AND    HEAT. 


165 


mirror  S ;  this  mirror  is  inclined  to  the  axis  of  the  tube  in  such  a  man- 
ner as  to  throw  the  inverted  image  a'  b'  down  to  the  focus  of  the  eye 
glass  o  o. 

The  speculum  of  Herschel's  largest  telescope  was  4  feet  in  diameter, 
with  a  focal  distance  of  40  feet.  A  much  larger  one  has  recently  been 
constructed  by  LORD  ROSSE,  in  Ireland.  The  speculum  is  6  feet  in 
diameter,  with  a  focal  distance  of  54  feet.  The  diameter  of  the  tube  is 
7  feet,  its  length  is  56  feet.  The  whole  weight  is  over  14  tons. 


THE    CAMERA    OBSCURA. 

40.  The  Camera  Obseiira,  or  dark  chamber,  in  its  most  simple  form,  is 
nothing  more  than  a  dark  room  with  a  hole  in  the  window  shutter,  in 
which  is  placed  a  convex  lens  of  about  two  feet  focal  length.  A  sheet 
of  white  paper  is  placed  vertically  behind  the  lens,  at  its  focus,  and  then 
an  accurate  picture  of  all  the  objects  seen  from  the  window  will  be  de- 
picted upon  the  surface  of  the  paper,  which  delights  and  surprises  every 
person  that  beholds  it.  In  order  to  obtain  a  perfect  picture,  the  ground 
on  which  it  is  received  should  be  hollow, 
and  a  portion  of  the  sphere  whose  radius  is 
the  focal  distance  of  the  lens  ;  it  is  custom- 
ary, therefore,  to  make  this  ground  of  plas- 
ter of  paris. 

In  order  to  enable  a  person  to  copy  this 
picture,  it  should  be  received  upon  a  hori- 
zontal sheet  of  paper.  This  is  readily  ef- 
fected by  means  of  a  plane  mirror  C  D, 
(see  Fig.  59,)  placed  at  an  angle  of  45°,  to 
reflect  the  rays  down  upon  the  lens  A  B, 
which  throws  down  the  picture  upon  the 
horizontal  table  E  F  placed  in  the  focus  of 
the  lens.  The  draughtsman  introduces  his 
head  through  an  opening  made  in  one  side 

of  the  frame,  and  his  hand,  holding  the  pencil,  through  another  opening, 
care  being  taken  that  no  light  is  allowed  to  fall  upon  the  picture.  The 
application  of  the  camera  obscura  to  photography  has  rendered  it  one  of 
our  most  useful  optical  instruments. 


MAGIC    LANTERN. 


41.  The  magic  lantern  is  an  obvioxis  application  of  a  microscope.  L 
(Fig.  60)  is  a  powerful  lamp  in  the  focus  of  a  concave  mirror  M  N, 
placed  in  a  dark  lantern ;  A  B  is  a  fixed  tube  containing  a  hemispherical 


166 


NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 


illuminating  lens  A,  and  a  convex  lens  B ;  C  D  is  an  opening  between 
the  lenses  A  and  B,  for  receiving  the  sliders  on  which  the  pictures  are 


Fig.  60. 

painted  with  highly  colored  transparent  varnish.  The  light  of  the  lamp 
is  reflected  by  the  mirror  M  N  upon  the  lens  A,  which  further  concen- 
trates the  light  upon  the  picture  on  the  slider ;  and  this  picture  is 
thrown,  very  much  enlarged,  upon  the  screen  E  F,  placed  in  the  focus 
of  the  lens.  The  lens  B  is  fixed  in  a  sliding  tube,  so  that  by  pulling  it 
out  or  pushing  it  in,  a  distinct  picture  of  the  object,  on  the  slider,  may 
be  formed,  of  any  size,  within  certain  limits,  upon  the  screen  E  F. 

The  solar  microscope  is  merely  a  magic  lantern,  where  the  light  of  the 
sun  is  substituted  for  the  light  of  the  lamp. 

The   Stereoscope. 

42.  When  we  view  any  solid  object,  such  as  a  statue,  with  both  eyes, 
each  eye  sees  the  object  differently,  and  two  dissimilar  pictures  of  the 
object  are  painted  on  the  retina.  But  each  two  corresponding  points  of 
the  two  pictures  are  depicted  at  the  same  place  on  the  optic  nerve,  so 
that  the  eyes,  uniting  each  pair  of  points  in  succession,  give  the  brain 
the  impression  of  a  solid.  Now,  by  inverting  this  process,  that  is,  by 
making  two  pictures  of  a  solid,  as  seen  by  each  eye,  and  uniting  them 
upon  the  retinae  by  squinting,  so  that  the  one  picture  may,  as  it  were,  be 
laid  upon  the  other,  the  combined  pictures  will  give  to  the  mind  the 
impression  of  a  solid,  seen  exactly  as  in  nature.  This  forms  the  princi- 
ple of  the  stereoscope. 

Brewster's  Stereoscope. — This  instrument  is  represented  in  Fig.  61. 
A  and  B  are  two  eye  tubes,  containing  each  a  semi-lens,  with  their 
curved  sides  turned  towards  each  other,  so  that  by  looking  through  their 
edges,  objects  in  their  focus  are  so  refracted  that  the  one  picture  can  be 
placed  above  the  other. 

If  we  now  place  the  annexed  drawings  of  a  six-sided  pyramid  A  and 


LIGHT   AND    HEAT. 


167 


B  in  the  bottom  of  the  box  by  sliding  them  in  at  C  D,  and  look  into  the 
instrument,  with  the  right  eye  at  A  and  the  left  at  B,  we  shall  see  a 


Fig.  61. 


solid  pyramid  with  its  apex  rising  to  the  eye.    If  the  two  figures  had 
been  united  by  squinting,  they  would  have  produced  a  hollow  pyramid. 


Fig.  62. 


Fig.  63. 


Here  the  left  hand  drawing  is  the  view  which  the  pyramid  would 
present  to  the  left  eye,  and  the  right  hand  drawing  the  view  which  the 
pyramid  would  present  to  the  right  eye.  Any  solid  object  may  be 
treated  in  the  same  manner. 

PHENOMENA  OF  COLOR. 

43.  A  ray  of  solar  light  is  formed  by  the  union  of  seven 
different  colored  rays.  This  may  be  proved  analytically  as 
well  as  synthetically  — analytically  by  transmitting  a  ray  of 


168          NATURAL    AND    EXPERIMENTAL   PHILOSOPHY. 

white  light  through  a  glass  prism,  when  it  becomes  resolved 
into  seven  different  colored  pencils  of  light,  which  have  been 
called  the  prismatic  colors;  and  synthetically,  by  showing 
that  white  or  colorless  light  is  produced  by  the  union  of  the 
different  colored  pencils  of  light. 

Some  transparent  bodies  only  transmit  certain  colored  portions  of 
light,  as,  for  example,  common  bottle  glass  only  transmits  the  green  rays 
of  light ;  blue  glass  only  transmits  the  blue  rays ;  and  so  on. 

Nature  presents  us  with  a  magnificent  analysis  of  solar  light  in  the 
rainbow,  where  the  seven  prismatic  colors  may  be  distinctly  seen. 

The  surfaces  of  bodies  decompose  light  by  reflection.  The  surface  of 
a  rose  leaf  reflects  the  red  light,  and  absorbs  all  the  other  colored  rays ; 
the  surface  of  gold  reflects  the  yellow  light,  and  absorbs  all  the  other 
colored  rays  ;  and  so  on.  Thus  the  many-colored  tints  which  we  see  in 
the  objects  around  us  are  familiar  examples  of  the  analysis  of  light. 

THE    SOLAR    SPECTRUM. 

44.  Newton  first  decomposed  solar  light  by  means  of  a 
solid  piece  of  glass  bounded  by  three  plane  surfaces,  and 
commonly  called  the  prism.  The  success  of  this  experiment 
depended  upon  the  fact,  that  the  primary  or  simple  rays,  of 
which  pure  white  light  is  composed,  possess  different  degrees 
of  refrangibility.  He  conducted  the  experiment  in  the  fol- 
lowing manner :  — 

A  sunbeam  S  H  is  admitted  into  a  dark  room  through  a  hole  H  made 
in  a  window  shutter  E  F  ;  a  prism  A  B  C  is  interposed  so  that  the  ray 
• 

Ei 


Fig.  64. 


LIGHT   AND   HEAT.  169 

shall  pass  obliquely  through  two  faces,  and  be  refracted  by  both.  The 
refracted  ray  is  received  upon  a  sheet  of  white  paper  M  N,  and,  instead 
of  a  spot  of  white  light,  there  is  formed  upon  the  paper  an  oblong  col- 
ored surface  K  L,  composed  of  tlfe  seven  primary  tints,  called  the  pris- 
matic or  solar  spectrum,  as  shown  in  Fig.  64. 

These  different  colored  rays  do  not  admit  of  any  further  analysis ;  but 
on  causing  them  all  to  be  united,  the  seven  colors  disappear,  and  white 
light  is  again  formed.  White  light,  therefore,  is  a  mixture  of  seven  pri- 
mary rays  of  different  colors,  —  red,  orange,  yellow,  green,  blue,  indigo, 
and  violet.  The  separation  of  these  primary  or  simple  rays  from  one 
another,  depends  upon  a  difference  in  then-  refrangibility  in  passing 
through  the  prism ;  thus  the  violet  ray  is  most  refracted,  and  the  red 
ray  is  the  least  refracted. 

45.  The  different  portions  of  the  solar  spectrum  have 
three  distinct  properties,  in  relation  to  light,  heat,  and  chemi- 
cal action.  The  most  luminous  portion  is  at  the  middle  of 
the  yellow  light,  the  most  heating  at  and  beyond  the  red,  and 
the  greatest  chemical  intensity  is  found  to  be  between  the 
violet  and  indigo. 

Fig.  65  exhibits  these  relative  intensities  by  three  curved  lines,  one 
showing  the  curve  of  luminous  intensity,  another  the  heating  or  thermal 


Fig.  65. 

intensity,  and  the  third  the  chemical  intensity,  or  the  power  which  light 
has  in  effecting  chemical  changes. 

46.  Brewster  considers  that  white  solar  light  is  composed 
of  only  three  primary  rays,  viz.,  red,  yellow,  and  blue ;  for 
the  admixture  of  these  three  colored  rays  will  produce  white 
light,  as  shown  in  Fig.  66.  These  are  called  the  three  fun- 
damental colors. 

Each  of  the  seven  prismatic  rays  has  some  other  colored 
15 


170 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


ray,  called  its  complementary  ray,  with  which  if  it  be  com- 
bined, white  light  will  be  produced. 

Fig.  66  shows  the  three  fundamenfal  colors,  red,  yellow,  and  blue, 
overlapping  each  other.     Where  all  three  overlap  one  another,  white 
is  produced  ;  where  the  yellow  and  blue 
overlap,  green  is  produced,  and,  therefore, 
green  and  red  will  produce  white  light, 
so  that  green  and  red  are  complementary ; 
and  so  on  to  other  cases  which  may  be 
readily  cited    from    the    representation 
given  in  the  figure.     Orange,  violet,  and 
green,  according  to  Brcwster,  are  called 
secondary  colors ;  while  red,  yellow,  and 
blue,  are  the  only  primary  colors;  and 
the  indigo  of  Newton's  spectrum  is  sup- 
posed to  lie  between  the  shades  of  the  Fig.  66. 
violet  and  the  blue. 

THE    RAINBOW. 

47.  The  brilliant  colors  of  dew  drops,  produced  by  the  refrac- 
tion of  the  sunbeams,  form  a  subject  of  interest  to  every  per- 
son. The  beautiful  arch  of  the  rainbow,  depending  upon  the 
same  cause,  is  not  less  a  matter  of  interest  to  even  the  most 
uneducated  observer.  The  formation  of  the  rainbow  may  be 
readily  explained  on  the  principle  of  prismatic  refraction  and 
dispersion. 

The  drops  of  rain  decompose  the  sun's  light  in  the  same  manner  as 
the  prism  of  glass.     Let  D  represent  a  drop  of  rain,  (see  Fig.  67  :)  a  6, 
a  ray  of  light  falling  upon  the  drop,  is  refracted  in  the 
direction  be;  it  is  then  reflected  in  the  direction  c  d, 
and  upon  passing  out  of  the  drop  at  d  it  undergoes  the 
prismatic  dispersion :  the  red  ray,  being  the  least  re- 
fracted, takes  the  lowest  direction  d  r,  and,  the  violet 
ray,  being  the  most  refracted,  takes  the  highest  direc- 
tion  d  v ;  hence  arise  the  prismatic  colors.     Now,  in 
Fig.  68,  let  D  represent  the  same  drop,  and  D'  another 
drop  a  little  below  the  first ;  then  the  same  prismatic 
colors  will  be  produced  by  this  second  drop,  and  at  some  -point  o  the 
red  ray  of  the  first  drop  will  meet  the  violet  ray  of  the  second  drop ; 
and  a  spectator,  with  his  eye  at  o,  will  see  the  red  ray  from  the  first 


LIGHT   AND    HEAT. 


171 


drop  and  the  violet  ray  from  the  second  drop,  and  from  the  drops  lying 
between  these  two  extremes  he  will  see  the  intermediate  prismatic  colors, 


Fig.  68. 

and  therefore  between  d  and  d'  he  will  see  a  complete  spectrum.  Now, 
let  s  u  be  a  straight  line  passing  through  the  centre  of  the  sun  and  the 
eye  of  the  observer  at  o,  which  will  of  course  be  parallel  to  the  rays  inci- 
dent upon  the  drops.  Conceive  the  angles  d  o  u  and  d'  o  u  to  be  turned 
about  o  u  as  an  axis :  then  the  drops  D  and  D'  will  revolve  in  a  circle, 
and  within  this  circle  all  the  prismatic  colors  will  obviously  be  arranged 
in  the  same  order  as  that  which  we  have  just  described ;  hence  the  pris- 
matic colors  will  appear  to  arrange  themselves  in  this  arch,  which  is 
called  the  primary  rainbow. 

The  secondary  rainbow  is  a  fainter  arch,  fre- 
quently lying  exterior  to  the  primary  one.  The  for- 
mation of  this  secondary  arch  may  be  explained 
exactly  in  the  same  manner,  with  this  exception, 
that  the  refracted  light  undergoes  two  reflections 
within  the  drop  in  the  place  of  one,  as  shown  in  Figs. 
68  and  69. 


Fig.  69. 


UNUSUAL  REFRACTION  OF  LIGHT,  AND  ATMOSPHERIC 
PHENOMENA  DEPENDING  UPON  IT. 

REFRACTION    OF   A   FLUID    OF    VARYING   DENSITY. 

48.  Into  a  square  vial  (Fig.  70)  pour  some  clear  sirup,  and  above  pour 
some  clear  water,  which  will  gradually  mix  with  the  sirup ;  hold  a  card, 
with  the  word  sirup  written  on  it,  in  an  erect  position,  behind  the  vial ; 
then  the  writing  will  appear,  in  its  erect  position,  when  seen  through  the 


172 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  70. 


Fig.  71. 


pure  sirup,  but  it  will  appear  inverted  when  seen  through  the  mixture 
of  sirup  and  water.  A  similar  phenomenon  will  be  produced  by  pouring 
spirits  of  wine  upon  the  water,  as  shown  in  the  figure. 

This  simple  experiment  will  enable  us  to  account  for  some  curious 
cases  of  unusual  atmospheric  refraction,  or  mirage. 

Sometimes  two  distinct  images  of  a  ship,  whose  topmast  A  only  is  seen 
above  the  horizon,  will  in  certain  states  of  the  atmosphere  appear  in  the 
air  as  represented  in  Fig.  71,  where  one  image  C  is  erect,  and  the  other 
B  is  inverted. 

In  order  to  account  for  these  appearances,  let  S  P  (Fig.  72)  represent 


Fig.  72. 

the  object ;  E  the  eye  of  the  observer ;  p  and  p'  the  images  seen  in  the 
air.  Now,  the  coldness  of  the  sea  may  cause  the  air  at  the  level  a  to  be 
very  much  denser  than  the  air  at  the  level  c  or  d;  in  this  case,  the  re- 


LIGHT   AND    HEAT. 


173 


fractivc  power  at  c  or  d  will  be  much  less  than  at  a ;  the  consequence  of 
this  is,  that  rays  S  d  P  c  \vhich,  under  a  uniform  state  of  density  of  the 
air,  never  would  reach  the  eye  at  E,  will  be  bent  into  the  curve  lines 
S  d  E,  P  c  E,  in  passing  from  the  rave  to  the  dense  medium  ;  and  if  the 
difference  of  density  is  such  that  the  higher  rays  S  d  cross  the  lower  rays 
P  c  at  any  point  x,  then  the  higher  rays  will  be  seen  in  the  direction  E  s, 
(where  E  5  forms  a  tangent  to  the  curve  S  d  x  E  at  the  point  E,)  and  the 
lower  rays  in  the  direction  E  p  ;  and  thus  the  image  of  the  ship  will  be 
seen  inverted  in  the  air.  In  like  manner  the  rays  S  n,  P  mt  may  be  re- 
fracted to  meet  the  eye  E  without  crossing  each  other ;  then  the  higher 
rays  S  n  E  will  be  seen  in  the  direction  E  s',  and  the  lower  rays  P  m  E 
will  be  seen  in  the  direction  E  p't  and  thus  the  image  of  the  ship,  in  this 
case,  will  be  seen  in  its  erect  position  p'  s'.  The  state  of  the  air  may  be 
such  as  to  exhibit  only  one  of  these  images. 

49.  The  subject  of  halos  may  be  ranked  amongst  the  opti- 
cal phenomena  of  the  atmosphere.  The  name  halo  is  given 
to  all  those  luminous  appearances  which  are  seen  surrounding 
the  sun  or  the  moon. 

One  of  the  most  common  phenomena  of  this  kind  is  the  divergence 
of  the  solar  beams,  represented  in  Fig.  73. 


Fig.  73. 

This  phenomenon  frequently  occurs  in  summer,  when  the  sun  is  near 
the  horizon.  It  is  caused  by  certain  portions  of  the  sun's  beams  radiat- 
ing through  the  openings  of  the  surrounding  clouds,  while  other  portions 
of  his  beams  are  obstructed  by  the  denser  parts  of  the  clouds. 


174          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


DOUBLE    REFRACTION. 

50.  Some  crystals  possess  the  curious  property  of  double 
refraction —  that  is,  of  making  one  object  appear  double. 

The  most  common  crystal  of  this  kind  is  Iceland  spar,  which  usually 
has  the  shape  of  a  solid  rhomb,  or  six-sided  parallelepiped  A  B  D  C  X, 
represented  in  Fig.  74. 

Place  a  rhomb  of  Iceland  spar 
over  a  black  line  M  N  drawn  upon 
a  sheet  of  paper ;  look  at  this  line 
through  the  upper  surface  of  the 
crystal  with  the  eye  at  R:  then 
the  line  M  N  will  probably  appear 
double ;  if  it  does  not  at  the  first 
trial,  turn  the  crystal  round  until 
you  distinctly  see  two  black  lines 
in  the  place  of  one.  pt  74.  f 

Place  a  black  spot  at  O,  or  prick 
a  pin  hole  in  the  paper ;  the  spot  will  appear  double,  as  at  O  and  E ;  turn 
the  crystal  round,  and  the  two  images  will  be  seen  apart  from  each  other : 
the  one  E  will  appear  to  revolve  round  the  other  O.  The  ray  O  r  is 
called  the  ordinary  ray  of  refraction,  and  E  r  the  extraordinary  one. 

The  ray  of  light,  after  separation  into  two  distinct  pencils  in  this  man- 
ner, is  said  to  be  polarized.  These  polarized  rays  possess  certain  peculiar 
properties,  which  distinguish  them  from  the  ordinary  rays  of  light. 

POLARIZED   LIGHT. 

51.  Light  is  polarized  by  reflection. 

Let  A  B  C  D  and  a  5  c  d  be  two  glass  reflectors  having  their  backs 
coated  with  black  varnish  ;  place  them,  as  represented  in  the  figure,  so 
that  the  rays  of  light  II  Q  proceeding  from  the  candle  II  may  be  reflected 
from  the  mirror  A  B  C  D  in  the  line  Q,  P,  and  that  these  reflected  rays 
may  undergo  a  second  reflection  from  the  other  mirror  a  b  c  d  in  the  line 
P  E.  Now,  if  the  two  mirrors  were  both  placed  vertically,  the  reflection 
of  the  light  of  the  candle  from  the  second  mirror  would  suffer  very  little 
or  no  diminution  ;  but  when  the  plane  of  the  second  mirror  is  placed  at 
right  angles  (or  nearly  at  right  angles)  to  the  plane  of  the  first  mirror, 
the  image  of  the  candle  reflected  from  the  second  mirror  is  so  dim  that 
it  can  scarcely  be  distinguished ;  and  if  the  reflections  are  made  at  the 


LIGHT   AND    HEAT. 


175 


''          '       "'•    !  ''.!'.•!•     "•:: ;_ ^_ 


Fig.  75. 

proper  angles,  the  ray  Q  P  will  not  be  at  all  reflected  from  the  mirror 
abed.  The  reflected  ray  Q  P  is  said  to  be  polarized  by  reflection.  The 
polarizing  angle  of  glass  is  about  56°,  that  of  water  is  about  52°,  and  so 
on  to  other  reflecting  surfaces. 

Having  placed  the  mirror  A  B  C  D  so  that  the  ray  Q  P  shall  be  re- 
flected at  the  angle  of  56°,  elevate  or  depress  the  angle  of  the  second 
mirror  abed  until  you  have  hit  upon  that  position  where  the  image  of 
the  candle  just  vanishes;  then  you  will  find  this  angle  to  be  about  56°, 
the  polarizing  angle  of  glass  at  which  the  polarized  ray  will  not  undergo 
a  second  reflection.  Blow  upon  the  glass  ;  the  light  will  appear  again. 
Why  ?  Because  the  polarizing  angle  of  water  is  different  from  that  of 
glass ;  the  moisture  on  the  glass  soon  disappears  by  evaporation,  and  then 
the  image  of  the  candle  again  vanishes. 

52.  Fig.  76  represents  an  instrument  constructed  on  the  principle  just 
explained.  C  D  and  D  G  are  brass  tubes,  the  one  capable  of  sliding 


Fig.  76. 

and  turning  within  the  other ;  A  and  B  -are  the  glass  mirrors  fixed  to 
the  two  tubes  at  the  polarizing  angles  ;  R  •;•  an  incident  ray  of  common 
light ;  r  s  the  line  of  its  reflection  from  the  mirror  A  through  the  tube ; 
s  E  the  line  of  reflection  from  the  mirror  B  ;  then,  when  the  tube  D  Gr 
is  turned  round  so  that  the  plane  of  the  mirror  B  is  at  right  angles  to 


176         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


the  plane  of  the  mirror  A,  the  ray  r  s  will  not  suffer  reflection  from  B. 
When  the  tube  D  G  is  turned  round  so  as  to  bring  the  plane  of  the  mir- 
ror B  parallel  to  that  of  A,  then  the  reflected  ray  s  E  will  appear  of  its 
proper  or  usual  degree  of  brightness. 

Exp.  1.  Let  the  incident  rays  R  Q  (Fig.  75)  proceed  from  the  light 
of  the  window ;  place  a  thin  plate  of  mica  between  the  two  mirrors  so 
as  to  intercept  the  polarized  rays  Q  P ;  look  in  the  direction  E  P,  and 
you  will  perceive  the  dark  reflector  lighted  up  with  the  most  splendid 
colors,  more  especially  if  the  plate  of  mica  varies  in  its  thickness.  Turn 
the  mica  round,  and  the  colors  will  pass  through  all  the  changes  of  the 
prismatic  light.  A  similar  eifect  will  be  produced  by  turning  the  re- 
flector abed  round  upon  Q  P  as  an  axis. 

Exp.  2.  In  like  manner,  place  a  piece  of  glass,  to  which 
a  crystalline  structure  has  been  given  by  rapidly  cooling  it, 
between  the  reflectors  :  then  the  glass  will  present  a  bril- 
liant and  symmetrical  figure  having  the  appearance  rep- 
resented in  Fig.  77.  Turn  the  mirror  abed  round  on 
Q  P  as  an  axis,  until  it  becomes  parallel  to  the  other  mir- 
ror A  B  C  D  ;  then  the  colored  figure  of  the  crystalline 
glass  will  assume  another  perfect  form,  which  is  repre- 
sented in  Fig.  78,  the  colors  in  the  one  being  complement- 
ary to  those  in  the  other. 

These  crystalline  pieces  of  glass  may  be  got  at  any  phil- 
osophical instrument  maker's  shop ;  but,  without  incur- 
ring this  expense,  the  experiment  may  be  performed  in  the  following 
manner :  — 

Bind  a  few  square  plates  of  window  glass  together,  and  place  them 
between  the  mirrors,  as  in  the  last  experiment,  on  a  hot  metal  plate ; 
look  in  the  reflector  abed;  a  curious  progressive  change  will  be  seen 
to  take  place  in  the  glass  plates,  when  at  length  the  symmetrical  figure 
shown  in  Fig.  77  will  be  formed. 

53.   Light  is  polarized  by  a  series  of  ordinary  refractions. 

When  a  ray  of  light  E,  r  (Fig.  79) 
undergoes  refraction  through  a  series 
of  glass  plates  1,  2,  3,  .  .  .,  the  re- 
fracted ray  f  g  becomes  polarized,  and 
possesses  all  the  properties  which  have 
been  described  in  relation  to  the  polar- 
ized light  of  reflection.  The  incident 
angle  of  perfect  polarization  depends 
upon  the  number  of  the  plates ;  thus, 


Fig.  79. 


LIGHT   AND    HEAT. 


177 


•when  there  are  eight  plates,  the  incident  angle  is  about  79° ;  and 
when  there  are  twenty-four  plates,  the  incident  angle  is  about  GO0, 
and  so  on. 

Now,  since  a  bundle  of  glass  plates  acts  upon  light  in  the  same  man- 
ner as  the  polarizing  reflectors  used  in  the  apparatus,  Figs.  75  and  76, 
we  may  substitute  two  bundles  of  glass  plates  in  the  place  of  the  two 
reflectors.    Thus,  let  A  and  B  be  the 
two  bundles  of  polarizing  plates ;  R  r 
the  incident  ray  ;  then  s  t  will  be  the     .j 
polarized  ray,  which  will  pass  through 

the  bundle  B  when  it  is  placed  as  in  pigt  so. 

the  figure,  and  no  light  will  be  re- 
flected ;  but  when  it  is  turned  round,  the  light  v  ^o  transmitted  through 
it  will  gradually  diminish,  and  more  and  more  light  will  be  reflected,  till 
it  has  turned  round  an  angle  of  90°  :  then  there  will  be  no  light  trans- 
mitted —  it  will  be  entirely  reflected. 

In  conducting  many  experiments,  p,  bundle  of  glass  plates  may  be 
advantageously  used  in  the  place  of  the  reflector  abed  of  the  appara- 
tus represented  in  Fig.  75. 

54.  Polarized  Light.  —  According  to  the  corpuscular  theory,  a  beam 
of  common  light  has  two  polar  axes,  A  B  and  C  D,  Fig.  81,  so  that  all 


Fig.  81. 

• 

its  sides  have  the  same  properties  ;  but  when  this  beam  is  polarized,  it  is 
separated  into  two  circular  beams,  A'  B'  and  C'  D',  with  only  one 
polar  axis  each,  which  are  at  right  angles  to  each  other,  so  that  their 
sides  have  different  properties. 


HEAT. 

55.  The  sense  of  touch  is  affected  by  heat,  as  our  sense  of 
hearing  is  by  sound,  or  our  sense  of  sight  by  light.  Heat  is 
one  of  the  most  important  agents  connected  with  animal  and 


178          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

vegetable  life ;  it  also  performs  a  distinguished  part  in  all  the 
changes  continually  going  on  in  the  external  world. 

Free  or  sensible  heat  tends  to  diffuse  itself  equally  among 
all  surrounding  bodies.  The  amount  of  sensible  heat  in  any 
body  is  called  its  temperature.  That  heat  which  exists  in  a 
body,  and  which  is  not  sensible  to  the  touch,  but  which  is,  at 
the  same  time,  essential  to  the  peculiar  form  in  which  the 
body  exists,  is  called  latent  heat. 

The  word  caloric  is  used  to  express  the  substance  of  heat, 
in  order  to  distinguish  it  from  the  sensation  of  heat.  We 
experience  the  sensation  of  heat  when  there  is  an  increase  of 
temperature,  and  that  of  cold  when  there  is  a  decrease  of 
temperature.  The  sensation  of  cold  is  excited  when  a  portion 
of  our  caloric  is  taken  from  us,  and  that  of  heat  when  a  por- 
tion of  caloric  is  transmitted  to  us. 

Caloric,  or  the  matter  of  heat,  is  subject  to  the  same  laws 
of  radiation,  reflection,  and  refraction  as  light.  Heat  pro- 
duces many  chemical  changes ;  it  also  tends  to  destroy  the 
cohesion  of  the  particles  composing  a  body,  and  thus  produces 
a  change  in  the  form  of  bodies ;  thus,  at  a  certain  low  tem- 
perature liquid  water  becomes  solid  ice,  and  at  a  certain  high 
temperature  it  boils  and  passes  into  the  state  of  vapor  or 
steam.  One  of  the  most  striking  effects  of  heat  is,  that  it 
causes  all  bodies  to  expand  —  that  is,  to  increase  in  volume 
or  bulk. 

EASY    COURSE    OF   EXPERIMENTS,   WITH    SIMPLE   PRINCIPLES 
DERIVED    FROM    THEM. 

56.    Heat  expands  liquids,  air,  and  solids. 

Exp.  1.  Heat  expands  liquids.  —  Take  a  common  vial  bottle;  make 
a  mark  with  ink  upon  its  neck ;  fill  it  with  cold  water  up  to  this  mark  ; 
plunge  the  vial  into  a  basin  of  hot  water  :  after  a  little  time  the  water 
in  the  vial  will  rise  considerably  above  the  mark,  thereby  showing  that 
the  heat  has  caused  the  water  in  the  vial  to  expand.  The  higher  the 
temperature  of  the  water  in  the  basin  the  greater  will  be  the  expansion 
of  the  water  in  the  vial. 


LIGHT    AND    HEAT. 


179 


This  experiment  shows  the  principle  upon  which  the  thermometer  is 
constructed.  This  well-known  instrument  enables  us  to  tell  the  tem- 
perature of  any  body. 

This  experiment  may  be  performed  in  a  more  strik- 
ing manner  as  follows  :  Fit  a  small  glass  tube  to  the 
cork  of  a  bottle,  as  shown  in  Fig.  82  ;  fill  the  bottle 
completely  with  cold  water,  and  firmly  insert  the  cork 
with  its  tube ;  plunge  the  bottle  into  hot  water,  and 
the  liquid  will  rapidly  rise  in  the  small  tube. 

Exp.  2.  Heat  expands  air.  —  Invert  a  glass  A  over 
water,  allowing  a  little  water  to  enter  the  glass,  as 
shown  in  Fig.  83 :  pour  hot  water  over  the  glass, 
which  will  cause  the  air  within  the  glass  to  expand, 
and  to  occupy  a  larger  space. 

Exp.  3.  Invert  a  small  bottle  in  cold  water,  and 
introduce  just  so  much  water  as  will  cause  it  to  sink 
to  the  bottom ;  now  pour  hot  water  into  the  vessel,  so 
as  to  raise  the  temperature  of  the  air  within  the  bottle  : 
the  bottle  will  rise  to  the  surface. 

The  variation  of  heat  in  the  atmosphere  is  the  cause 
of  currents  of  air  and  winds.  This  has  been  explained 
in  Pneumatics. 

Exp.  4.  Perform  experiments  1  and  2,  given  at 
page  107. 

Exp.  5.  Heat  expands  solids. — Take  a  decanter 
bottle,  having  a  ground  stopper ;  plunge  the  neck  of 
the  decanter  into  hot  water,  and  there  let  it  remain  for  a  short  time ; 
after  taking  it  out,  insert  the  stopper  gently,  so  that  it  may  be  easily 
taken  out ;  allow  the  neck  of  the  decanter  to  cool,  then  try  to  raise  the 
stopper ;  it  will  have  become  so  fast,  from  the  contraction  of  the  glass, 
that  it  requires  some  force  to  pull  it  out.  Here  the  heat  causes  the 
neck  of  the  decanter  to  expand  ;  then,  when  the  stopper  is  put  into  its 
place,  the  neck  of  the  decanter,  as  it  cools,  contracts  upon,  the  stopper, 
and  causes  it  to  become  fast. 

If  you  should  happen  to  heat  the  neck  of  the  decanter  so  much  that 
you  cannot  pull  the  stopper  out,  (a  thing  not  at  all  unlikely  to  happen,) 
then  you  must  get  it  out  by  the  same  means  as  that  by  which  you  fas- 
tened it  in ;  that  is,  you  must  heat  the  neck  of  the  bottle,  so  as  to  cause 
it  to  expand. 

Many  a  good  bottle  has  been  broken  by  hastily  putting  in  the  cold 
stopper  when  the  bottle  was  warm. 

57.    Sources  of  Heat.  —  Besides  the  heat  derived  from  the 


Fly,  83. 


180         NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 


Fig.  84. 


Fig.  85. 


sun,  we  get  heat  from  our  ordinary  fires,  lamps,  from  the 
friction  and  collision  of  bodies,  from  chemical  action,  &c. 

Exp.  1.  Heat  from  common  flame.  —  (a.)  The  degree  of  heat  of  a 
flame  depends  upon  the  supply  of  air.  Place  a 
common  lamp  glass  over  a  lighted  candle ;  the 
candle  will  burn  very  feebly,  unless  you  raise  the 
glass  a  little  so  as  to  admit  a  current  of  air 
through  the  tube.  (6.)  Slowly  and  gradually  in- 
sert a  burning  splinter  of  dry  wood  into  a  small 
vial  bottle ;  the  portion  of  the  splinter  outside 
of  the  bottle  burns,  but  that  within  the  vial 
merely  becomes  carbonized,  because  there  is  not  a 
sufficient  quantity  of  air  to  burn  it  completely. 
On  this  principle  charcoal  is  made. 

Exp.  2.  Heat  from  friction.  —  Rub  a 
button  upon  a  deal  board ;  the  button 
will  soon  become  quite  hot. 

Exp.  3.  Heat  from  collision,  (a.) 
Strike  a  spark  with  a  flint  and  a  steel. 
(6.)  Hammer  a  piece  of  iron  until  it  be- 
comes hot. 

Exp.  4.  Heat  from  chemical  action.  —  («.)  Place  a  small  bit  of  phos- 
phorus upon  a  dry  deal  board ;  drop  a  small  piece  of  iodine  upon  the 
phosphorus ;  the  bodies  will  unite  spontaneously,  and  will  form  a  com- 
pound of  iodine  and  phosphorus. 

(6.)  Pour  seme  water  upon  sulphuric  acid ;  the  mixture  will  become 
intensely  hot.  In  this  case,  the  volume  of  the  mixture  will  be  less  than 
the  sum  of  the  volumes  of  the  two  liquids.  This  condensation  of  vol- 
ume is  no  doubt  the  cause  of  the  development  of  the  heat,  for  a  change 
of  volume  is  always  attended  with  a  change  of  specific  heat,  or  a  change 
of  the  body's  capacity  for  heat. 

58.    Good  and  Bad  Conductors  of  Heat. 

Exp.  1.  Put  the  end  of  a  tobacco  pipe  into  the  fire,  and  at  the  same 
time  put  the  end  of  a  poker  into  the  fire  ;  after  the  lapse  of  a  few  min- 
utes, touch  the  poker,  at  the  distance  of  a  few  inches  from  the  heated 
extremity,  and  it  will  feel  quite  hot ;  at  the  same  moment  touch  the 
extremity  of  the  tobacco  pipe,  and  it  will  scarcely  feel  warm :  thus 
showing  that  iron  is  a  much  better  conductor  of  heat  than  the  material 
composing  the  pipe. 

Exp.  2.   Touch  the  metal  portion  of  the  handle  of  an.  Italian  iron ; 


~IGHT    AND    HEAT.  181 

it  will  feel  hot ;  touch  the  wooden  portion  of  the  handle,  and  it  will  feel 
comparatively  cool ;  thereby  showing  that  iron  is  a  much  better  con- 
ductor of  heat  than  wood. 

Compare  the  heat  of  the  handle  of  a  saucepan  having  a  metal  handle, 
with  the  heat  of  the  wooden  handle  of  another  saucepan. 

Exp.  3.  Touch  the  -wooden  leg  of  a  table  with  one  hand,  and  the 
brass  castor  with  the  other  ;  the  one  feels  cold,  the  other  neither  hot  nor 
cold.  Here  the  metal,  being  a  good  conductor  of  heat,  conveys  the  heat 
from  the  hand  more  rapidly  than  the  wood,  which  is  a  bad  conductor  of 
heat. 

Exp.  4.  Touch  the  hot  surface  of  a  teapot  through  a  piece  of  paper ; 
you  scarcely  feel  the  heat :  now  touch  it  through  a  piece  of  tin  foil  or 
sheet  lead  ;  you  instantly  feel  the  heat.  The  paper  is  a  bad  conductor 
of  heat ;  but  the  tin  foil,  as  well  as  metals  generally,  is  a  good  conductor 
of  heat. 

We  clothe  our  bodies  with  woollen  and  linen,  and  such  like  materials, 
because  they  are  bad  conductors  of  heat. 

Exp.  5.  Fill  a  common  porter  bottle  with  hot  water,  and,  after  cork- 
ing it  up,  wrap  it  in  a  dry  piece  of  flannel ;  the  bottle  may  remain  in 
that  state  for  an  hour,  without  much  sensible  change  in  its  heat.  Here 
the  heat  is  kept  in  the  bottle  by  the  non-conducting  substance  with 
which  it  is  surrounded. 

Exp.  6.  Pour  some  cold  water  into  a  tumbler ;  carefully  potu*  some 
hot  water  upon  the  top  of  the  other ;  apply  your  hand  to  the  lower 
part  of  the  tumbler  :  the  temperature  of  the  water  beneath  is  scarcely  at 
all  affected  ;  thereby  showing  that  water  and  glass  are  both  bad  con- 
ductors of  heat,  and,  moreover,  that  the  hot  water  is  lighter  than  the 
cold  water. 

Hot  water  is  specifically  lighter  than  cold  water,  because  of  the  ex- 
pansion by  heat,  which  causes  bodies  to  become  less  dense. 

Try  to  place  cold  water  on  the  top  of  hot  water. 

Exp.  7.  Nearly  fill  a  tumbler  with  cold  water ;  pour  some  ether  upon 
its  surface ;  the  ether  will  float  upon  the  water :  ignite  the  ether  by 
throwing  a  small  piece  of  lighted  paper  upon  it ;  the  great  heat  at  the 
surface  of  the  water  will  not  sensibly  affect  the  temperature  of  the  water 
at  the  lower  portion  of  the  tumbler. 

Exp.  8.  Take  two  pieces  of  small  wire,  of  exactly  the  same  length 
and  thickness,  the  one  being  copper  wire,  and  the  other  iron  or  steel 
wire  ;  hold  one  in  each  hand,  and  insert  their  extremities  into  the  flame 
of  a  candle ;  you  will  find  that  the  heat  will  pass  along  the  copper  wire 
much  more  rapidly  than  it  will  pass  along  the  iron  wire,  for  the  con- 
ducting power  of  copper  is  more  than,  double  that  of  iron. 
16 


182  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

59.    Good  and  Bad  Radiators  and  Reflectors  of  Heat. 

Exp.  1.  Place  a  tin  plate  and  a  piece  of  board  -within  a  foot  and  a 
half  of  a  good  fire ;  after  a  few  minutes,  the  surface  of  the  deal  board 
•will  feel  quite  hot,  but  the  temperature  of  the  tin  will  scarcely  at  all  be 
altered.  What  is  the  cause  of  this  remarkable  effect  r  Wood  is  a  bad 
reflector  of  heat,  and  therefore  it  absorbs  nearly  all  the  heat  which  falls 
upon  it ;  on  the  other  hand,  tin  plate  is  an  excellent  reflector  of  heat, 
and  therefore  nearly  all  the  rays  of  heat  which  fall  upon  its  surface  are 
reflected  from  it. 

Exp.  2.  Observe,  when  the  sun  is  shining,  that  the  panes  of  the  win- 
dow never  become  warm,  while  the  wooden  bars  become  hot. 

Exp.  3.  Hold  a  tin  plate,  in  an  inclined  position,  a  few  feet  before  a 
good  fire ;  receive  the  reflected  heat  upon  the  hand ;  you  will  feel  a  de- 
cided increase  of  temperature. 

Exp.  4.  Try  the  same  experiment  with  a  deal  board,  or  with  a  rough 
plate. 

Exp.  5.  Take  a  clean  metal  teapot,  and  a  common  earthen  ware  one  ; 
fill  them  both  with  hot  water ;  allow  them  to  stand  for  about  a  quarter 
of  an  hour ;  dip  your  hand  into  the  water  of  each ;  the  water  in  the 
metal  teapot  feels  much  warmer  than  that  which  is  in  the  earthen  ware 
one.  Why  ?  Simply  because  the  earthen  ware  vessel  is  a  much  better 
radiator  of  heat  than  the  metal  one. 

The  principle  which  regulates  the  power  of  radiating  sur- 
faces is  this :  The  best  reflectors  are  the  worst  radiators. 
Thus,  bright  polished  surfaces,  (other  things  being  the  same,) 
•which  are  the  best  reflectors,  are  the  worst  radiators ;  and 
rough,  black  surfaces,  which  are  the  worst  reflectors,  are  the 
best  radiators.  Bad  reflectors  either  transmit  or  absorb  the 
heat  which  falls  upon  them. 

Exp.  6.  Cover  half  of  one  side  of  a  piece  of  glass  with  tin  foil ;  hold 
the  covered  side  next  to  a  good  fire  ;  place  your  hand  on  the  other  side  ; 
no  heat  will  be  felt  on  that  part  of  the  glass  which  is  behind  the  tin  foil, 
but  a  sensible  temperature  will  be  felt  behind  the  other  portion :  here 
the  tin  foil  reflects  all  the  heat,  and  the  glass  transmits  a  portion  of  heat 
through  it. 

Now  blacken  the  uncovered  portion  of  the  glass  with  soot,  and  a  still 
greater  difference  of  heat  will  be  observed.  In  this  case,  the  soot  ab- 
sorbs all  the  heat  which  falls  upon  it,  and  becoming  thereby  heated, 
radiates  this  heat  to  the  hand. 

Exp.  7.   Envelop  two  tumblers  with  paper,  one  with  black  paper,  the 


LIGHT   AND    HEAT.  183 

other  with  silver  paper  ;  partly  fill  the  tumblers  with  water,  an  expose 
them  to  the  heat  of  the  sun.  ;  after  the  lapse  of  a  few  minutes,  ascertain 
the  temperature  of  the  water  in  the  tumblers-,  by  means  of  a  thermom- 
eter ;  the  water  in  the  tumbler  with  the  black  paper  will  be  found  to  be 
much  warmer  than  the  water  in  the  other.  Here,  the  black  paper  ab- 
sorbs the  heat,  while  the  silver  paper  reflects  it. 

Reverse  the  form  of  this  experiment  by  filling  the  tumblers  with  hot 
water  ;  after  the  lapse  of  a  few  minutes,  the  water  in  the  tumbler  with 
the  black  paper  will  be  fowtod  to  be  much  cooler  than  the  water  in  the 
other. 

Here  the  black  paper  radiates  the  heat  much  more  rapidly  than  the 
silver  paper. 

Exp.  8.  Make  two  little  fire  screens,  one  of  pasteboard,  and  the  other 
of  tin  plate ;  place  them  about  a  foot  before  the  fire,  and  after  a  few 
minutes  try  the  heat  which  they  transmit ;  the  air  beyond  the  paste- 
board will  be  much  warmer  than  that  which  lies  beyond  the  tin 
plate. 

60.  Heat  changes  liquids  into  vapors,  and  cold  condenses 
these  vapors. 

Exp.  1.  When  water  boils  in  the  kettle,  observe  the  steam  or  vapor 
as  it  issues  from  the  spout.  i 

(a)  The  vapor  is  seen  for  about  an  inch  in  front  of  the  spout ;  it  then 
rises  and  gradually  disappears  by  mixing  with  the  air.  The  air,  it  must 
be  observed,  can  always  absorb  or  retain  a  certain  portion  of  vapor. 

(6)  Hold  a  cold  plate  in  front  of  the  steam  ;  it  is  condensed,  that  is 
to  say,  it  is  converted  into  water  again.  In  a  short  time  the  plate  will 
become  quite  hot,  from  the  heat  given  up  by  the  steam  on  its  return  to 
the  liquid  state.  This  heat  is  called  latent  heat,  because  the  water  after 
condensation  has  the  same  temperature  as  it  had  just  before  condensa- 
tion :  this  latent  heat  is  the  heat  requisite  for  maintaining  water  in  the 
state  of  steam  or  vapor.  Whenever  a  body  passes  from  the  vaporous 
state  to  the  liquid  state,  or  from  the  liquid  state  to  the  solid,  it  must  give 
off  its  latent  heat,  and  vice  versa. 

(c)  Plunge  the  ball  of  a  thermometer  into  the  steam ;  the  mercury 
will  rise  in  the  small  tube  until  it  arrives  at  212°,  where  it  will  remain. 
Plunge  the  ball  into  the  boiling  water;  the  mercury,  as  before,  will 
stand  at  212°.  Water  under  ordinary  circumstances  constantly  boils  at 
a  temperature  of  212°.  '  What  becomes  of  all  the  heat  that  is  con- 
stantly passing  frqm  the  fire  to  the  water  ?  It  remains  in  a  latent  state 
in  the  steam.  So  long  as  water  remains  in  the  kettle  there  is  no  danger 
of  it  being  destroyed  by  the  heat ;  but  the  kettle  will  soon  be  cracked 
by  the  heat  if  it  is  allowed  to  remain  on  the  fire  after  the  water  has 


184          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

been  boiled  away.  The  evaporation  of  the  water,  by  constantly  ab- 
sorbing the  heat,  prevents  the  metal  from  rising  above  a  certain  degree 
of  heat. 

(d)  Observe  the  violent  ebullition,  or  boiling  up,  of  the  water,  as  the 
steam  issues  from  its  surface. 

Exp.  2.  Boil  some  water  in  an  egg  shell ;  the  evaporation  of  the 
water  prevents  the  egg  shell  from  being  burned. 

In  warm,  dry  weather,  water  rises  spontaneously  into  the 
air  ;  this  is  called  evaporation. 

Exp.  3.  Pour  a  little  water  on  a  plate  ;  after  a  short  time,  if  the  at- 
mosphere is  in  a  dry  state,  all  the  water  will  be  evaporated.  Where  has 
it  gone  ?  It  is  absorbed  by  the  surrounding  air,  which  has  a  certain 
capacity  for  retaining  moisture ;  this  capacity  increases  with  the  tem- 
perature of  the  air. 

A  drop  of  ether,  let  fall  upon  a  plate,  will  be  still  more  rapidly  evap- 
orated. 

Wrap  a  bit  of  blotting  paper  round  the  ball  of  a  thermometer; 
moisten  the  paper  with  water  ;  in  a  short  time  the  mercury  in  the  tube 
will  fall ;  thereby  showing  that  the  evaporation  of  the  water  produces 
cold. 

The  effect  in  this  experiment  will  be  more  marked  if  spirits  or  ether 
are  used  in  the  place  of  water. 

Let  fall  a  drop  of  spirits  of  wine,  or  ether,  upon  the  back  of  the  hand ; 
move  the  hand  backwards  and  forwards ;  the  liquid  will  be  quickly 
evaporated,  and  a  sensation  of  cold  will  be  produced  on  that  part  of  the 
hand  where  the  drop  was  placed. 

When  the  air  is  cooled  down  to  a  certain  point,  it  deposits 
moisture ;  this  is  called  the  dew  point. 

Exp.  4.  Bring  a  cold  plate  from  the  external  air  into  a  warm  room, 
where  there  is  a  good  fire ;  moisture  will  be  instantaneously  deposited 
upon  the  plate.  Here  the  air  in  contact  with  the  cold  plate  deposits 
a  portion  of  its  moisture.  Take  -  a  dry  tumbler  into  a  warm  room  ; 
'  fill  the  glass  with  cold  spring  water  ;  moisture  will  be  deposited  upon 
the  outside  of  the  glass. 

The  temperature  of  the  water  just  requisite  for  forming  the  deposition 
•of  moisture  is  called  the  dew  point  of  the  air  in  the  apartment. 

Dew  is  formed  upon  the  leaves  of  the  plants  in  a  similar  way. 

When  air  contains  all  the  moisture  which  it  is  capable  of  supporting, 
it  is  said  to  be  saturated  with  moisture.  In  damp^veather,  the  ;iir  is 
always  saturated  with  moisture*  but  in  dry,  clear  weather,  the  sur  is 
usually  below  this  point  of  saturation. 


LIGHT    AND    HEAT.  185 

The  evaporation  of  fhoisture  from  the  earth  goes  on  more 
rapidly  during  warm,  dry  weather,  than  in  cold,  damp  weather. 
When  the  atmosphere  is  warm  and  dry,  the  moisture  in  it  is 
perfectly  invisible ;  but  when  the  atmosphere  undergoes  a 
great  reduction  of  temperature,  the  moisture  which  is  in  it 
becomes  visible,  and  is  deposited  in  the  form  of  fog,  or  mist, 
or  dew,  and  also  in  the  form  of  rain  or  snow.  The  absolute 
quantity  of  moisture  which  the  air  will  sustain  depends  solely 
upon  its  temperature  ;  but  the  process  of  evaporation  is  accel- 
erated by  the  rarefaction  of  the  air ;  that  is  to  say,  other 
things  being  the  same,  water  will  be  much  more  rapidly 
evaporated  in  an  atmosphere  of  low  pressure  than  in  an  at- 
mosphere of  high  pressure. 

Exp.  5.    Water  boils  at  a  low  temperature  in  a  vacuum.  —  Half  fill  a 
flask  with  hot  water  ;  boil  the  water. until  steam  issues  from  the  mouth  ; 
remove  the  flask  from  the  flame  and  quickly  cork 
it :    the  boiling  immediately  ceases.     Pour  cold 
water  over  the  upper  part  of  the  flask ;  the  boil- 
ing immediately  begins  again  with  increased  vio- 
lence. 

Here  cold  appears  to  make  the  water  boil ;  how 
is  this  ?  The  cold  water  condenses  the  steam  in 
the  upper  portion  of  the  flask,  and  forms  a  vacu- 
um, or  at  least  a  partial  vacuum,  and  the  water 
boils  again  because  it  is  not  subject  to  any  pressure 
upon  its  surface.  This  explains  why  water  boils 
at  a  less  temperature  upon  the  tops  of  mountains 
than  it  does  in  the  plains  or  valleys. 

When  the  lid  of  a  saucepan  is  kept  tightly  down,  the  water  boils  at 
a  higher  temperature  than  212°.  On  this  principle  we  obtain  steam  in 
the  boiler  of  the  steam  engine  of  a  great  expansive  pressure. 

61.    Cold  is  produced  when  certain  substances  melt. 

Exp.  1.  When  you  form  an  effervescing  draught,  observe  that  the 
drink  is  very  cold.  Plunge  the  bulb  of  a  thermometer  into  the  mixture, 
and  you  will  find  that  the  mercury  will  fall  several  degrees.  Here  two 
crystalline  substances  are  rapidly  dissolved  by  water;  and,  moreover, 
the  rapid  escape  of  carbonic  acid  gas  further  aids  the  reduction  of  tem- 
perature. 

16* 


186 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Exp.  2.  Mix  some  snow  or  ice  and  common  salt  together ;  the  two 
solids  will  become  a  liquid,  and  an  intense  degree  of  cold  will  be  pro- 
duced. This  is  called  a  freezing  mixture. 

Exp.  3.  Put  a  piece  of  ice  into  water ;  plunge  a  thermometer  into  the 
liquid  :  in  a  short  time  the  thermometer  will  sink  to  32°,  the  tempera- 
ture at  which  water  freezes  or  at  which  ice  melts  :  the  thermometer  will 
stand  at  32°  so  long  as  there  is  a  particle  of  ice  in  the  water. 


CERTAIN   LAWS  AND   PHENOMENA  OF   HEAT  MORE 
FULLY  CONSIDERED. 


EXPANSION    OF   BODIES    BY   HEAT. 

62.  In  the  construction  of  large  metal  structures,  a  due 
allowance  is  always  made  for  the  expansion  and  contraction 
of  the  material  from  the  changes  of  temperature.  In  the 
great  tubular  bridges,  several  inches'  play  are  allowed  for  the 
expansion  and  contraction  of  the  metal. 

The  following  apparatus  shows  in  a  striking  manner  the  expansion  of 
metals  by  heat :  — 

C  D  is  a  metal  rod  furnished  with  a  handle 
A ;  B  a  flat  plate,  pierced  with  a  hole,  into 
which  the  rod  C  D  fits  freely  and  exactly,  and 
provided  with  a  notch  in  one  side  exactly  cor- 
responding to  the  length  of  the  rod  C  D  when 
it  is  of  the  usual  temperature.  Now,  when  C  D 
is  heated  in  the  fire,  it  will  expand  in  all  direc- 
tions ;  and  it  will  be  found  to  be  too  thick  to 
enter  the  hole,  and  its  length  will  be  so  much 
increased  that  it  will  not  enter  the  notch.  Let 
the  bar  C  D  be  now  plunged  into  cold  water, 
then  it  will  return  to  its  original  dimensions 
and  it  will  again  fit  the  hole  and  the  notch. 


Fig.  87. 


LIGHT   AND    HEAT. 


187 


Compensation  Pendulums. 

63.  In  order  that  a  pendulum  should  exactly  vibrate 
in  the  same  time  in  winter  and  summer,  it  is  necessary 
that  its  length  should  not  be  altered  by  slight  variations 
of  temperature.  Pendulums  which  are  constructed  so  as 
to  counteract  the  influence  of  changes  of  temperature  are 
called  compensation  pendulums. 

The  gridiron  pendulum,  represented  in  Fig.  88,  consists 
of  two  different  kinds  of  metals,  connected  together  some- 
what in  the  form  of  a  gridiron.  The  bob  P  is  suspended 
by  the  iron  rod  P  C,  which  is  attached  to  the  two  zinc 
rods  F  G  and  K  L  terminating  at  the  bottom  in  the  iron 
frame  B  E  D  A.  Now,  under  equal  augmentations  of  heat, 
zinc  expands  about  twice  as  much  as  iron ;  hence,  if  the 
length  of  the  iron  rods  in  this  pendulum  be  about  double 
that  of  the  zinc  rods,  the  expansion  of  the  one  metal  would 
exactly  counteract  the  expansion  of  the  other.  The  ex- 
pansion of  the  iron  rod  C  P,  as  well  as  the  expansion  of 
the  iron  frame  A  D  E  B,  carries  the  bob  P  farther  away 
from  the  point  of  suspension  S ;  but  the  expansion  of  the 
two  zinc  rods  G  F  and  L  K  brings  the  bob  P  nearer  to  the 
suspension  S ;  and  when  these  two  expansions  are  equal, 
the  distance  between  the  bob  and  the  point  of  suspension 
remains  the  same ;  that  is,  the  length  of  the  pendulum  re- 
mains the  same  under  every  change  of  temperature. 

The  mercurial  compensation  pendulum,  represented  in 
Fig.  89,  is  a  more  simple  contrivance  for  attaining  the 
same  end.'  A  glass  vessel  B  containing  some  mercury  is 
suspended  from  the  pendulum  rod  A  C.  When  the  rod 
A  C  expands,  the  distance  of  the  vessel  B  from  the  point 
of  suspension  C  is  increased  ;  but,  on  the  other  hand,  the 
expansion  of  the  mercury  in  the  vessel  brings  the  centre 


Fig.  88. 


Fig.  89. 


of  gravity  of  the  mass  nearer  to  the  point  of  suspension ;  and  the  pro- 
portion of  the  parts  may  be  so  adjusted  that  the  effect  of  the  expansion 
in  one  direction  may  exactly  neutralize  the  effect  of  expansion  in  the 
contrary  direction. 

Thermometers. 

64.  These  important  instruments  are  used  to  measure  the 
degree  of  temperature  to  which  bodies  are  raised.  When  very 
high  temperatures  are  to  be  measured,  the  instrument  is  called 


188  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

a  pyrometer.  The  change  of  volume  which  takes  place  in 
the  substance  employed  in  the  instrument  serves  as  an  index 
to  the  degree  of  heat.  The  thermometer  derives  its  name 
from  the  particular  thermoscopic  substance  used ;  thus  we 
have  the  common  mercurial  thermometer,  the  spirit-of-wine 
thermometer,  and  the  air  thermometer. 

The  mercurial  thermometer  consists  of  a  small  glass  tube  A  C  of  uni- 
form bore,  to  the  end  of  which  a  bulb  B  is  blown  ;  this  bulb  and  a  small 
portion  of  the  stem  are  filled  with  quicksilver,  and  the  open  end  of  the 
tube  is  hermetically  sealed. 

Under  ordinary  circumstances,  water  always  boils  and  freezes  at  the 
same  temperature  :  this  gives  us  the  means  of  fixing  a  true  scale  of  com- 
parison for  all  thermometers.  In  our  country  the  freezing  temperature 
of  water  is  called  32°,  and  its  boiling  temperature  212°,  so 
that  between  these  two  points  of  the  scale  we  have  180  equal 
olivisions  or  degrees,  each  equal  portion  being  the  amount  of 
expansion  due  to  1°  of  temperature.  To  graduate  the  ther- 
mometer,  therefore,  we  first  plunge  the  bulb  into  freezing 
water,  or,  what  is  the  same  thing,  into  melting  ice,  and  place 
a  mark  of  32°  at  A,  opposite  to  the  point  at  which  the  mer- 
cury stands  in  the  tube  ;  we  then  plunge  the  bulb  into  boil- 
ing water,  and,  in  like  manner,  place  a  mark  of  212°  at  C, 
opposite  to  the  point  at  which  the  mercury  now  stands  in  the 
stem ;  the  distance  between  these  two  points  A  and  C  is  then 
divided  into  180  equal  parts,  each  part  being  called  a  degree, 
and  the  scale  is  extended  upwards  or  downwards  accordingly.  B|_^ 

The  spirit- of -icine  thermometer  is  graduated  in  a  similar      -&• 
manner. 

The  air  thermometer.  —  Fig.  91  represents  a  common  air 
thermometer.  Here  the  open  end  of  the  tube  d,  with  its  bulb 
b  uppermost,  is  inserted  in  some  colored  liquid,  which  is  al- 
lowed to  rise  up  a  portion  of  the  tube,  as  to  d.  When  heat  is 
applied  to  the  bulb  b,  the  air  in  it  expands,  and  thus  depresses 
the  liquid  d  in  the  tube  ;  from  the  amount  of  depression  an 
estimate  may  be  formed  of  the  degree  of  heat  applied  to  the 
bulb.  These  instruments  are  exceedingly  sensitive. 

The  differential  thermometer,  represented  in  Fig.  92,  consists 
of  two  glass  bulbs  a  a  containing  atmospheric  air,  but  the 
lower  one  is  partially  filled  with  a  colored  fluid  which  rises  in   Fig.  91. 
the  glass  tube  to  the  zero  point  0.    From  this  point  the  de- 
grees on  the  scale  run  up  and  down,  as  shown  in  the  figure.    It  will  be 


LIGHT    AND    HEAT. 


189 


understood  that  when  the  two  bulbs  are  placed  under  the  same  heat, 
as  they  usually  are  when  the  instrument  is  not  in  use,  the  colored  liquid 
stands  at  the  zero  point  0  on  the  scale ;  but  if  the  temperature  of  the 
upper  bulb  be  raised,  then  the  liquid  will  sink  below  this  zero  point,  and, 


Fig.  92. 


Fig.  93. 


on  the  contrary,  if  the  temperature  of  the  upper  bulb  be  lowered,  the 
liquid  will  rise  above  the  zero  point ;  hence  the  instrument  has  been 
called  the  differential  thermometer,  because  it  measures  any  minute  dif- 
ferences of  temperature  in  the  two  bulbs. 

Fig.  93  represents  another  form  of  this  instrument. 

PROPAGATION   OF   HEAT. 

65.  The  free  caloric,  as  already  stated,  in  all  bodies,  tends 
to  a  state  of  equilibrium,  or  to  a  state  of  equality,  with  respect 
to  its  distribution.  Heat  is  propagated  by  direct  radiation,  by 
reflection,  and  by  conduction. 


RADIATION    OF    CALORIC. 

66.  Radiant  caloric,  like  light,  is  thrown  off  from  the  sur- 
faces of  bodies  in  all  directions  in  right  angles. 

The  intensity  of  radiant  caloric  may  be  measured  in  the  following 
manner  :  — 

Exp.  Place  the  bulb  T  (Fig.  94)  of  an  air  thermometer  near  to  a  hot 
metal  ball  M  ;  between  them  interpose  the  screen  S,  through  which  an 
aperture  O  is  made. 

When  the  aperture  O  is  brought  in  a  line  with  the  heated  ball  M 
and  the  bulb  T  of  the  thermometer,  the  liquid  instantly  descends.  By 


190 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  94. 

placing  the  bulb  of  the  thermometer  at  different  distances  from  M,  the 
relative  intensities  of  the  radiated  heat  may  be  duly  ascertained. 

In  order  to  show  that  reflected  heat  follows  the  same  law  as  reflected 
light,  place  a  red  hot  ball  a  in  the  focus  of  the  concave  tin  reflector 
n  d  m  ;  the  rays  of  caloric  will  be  reflected  in  the  parallel  lines  n  p  m  0. 


v 


Fig.  95. 

&c.,  *and;  meeting  the  second  reflector  p  r  o,  they  will  be  reflected  to  the 
focus  6,  and  a  thermometer  placed  there  will  indicate  the  degree  of  heat 
reflected.  The  surface  of  these  reflectors  should  have  a  parabolic  form, 
in  order  that  all  the  parallel  rays  may  meet  in  the  same  focus. 

If  a  lump  of  ice  be  substituted  in  the  place  of  the  hot  ball,  the  ther- 
mometer in  the  focus  b  will  instantly  indicate  a  fall  in  the  temperature. 
In  this  case,  more  caloric  radiates  from  the  ball  of  the  thermometer  than 
from  the  lump  of  ice ;  the  consequence  is,  that  the  ball  of  the  thermom- 
eter suffers  a  diminution  in  the  quantity  of  its  heat. 

Exp.  Let  M  and  M'  (Fig.  96)  be  two  concave  parabolic  reflectors, 
placed  at  the  distance  of  ten  or  twelve  feet  from  each  other.  Place  some 


Fig.  96. 


LIGHT    AND    HEAT. 


191 


phosphorus  or  gunpowder  in  the  focus/  of  the  reflector  M,  and  a  red  hot 
metallic  ball  in  the  other  focus/' ;  in  a  few  minutes  the  phosphorus  or 
gunpowder  will  be  ignited  by  the  heat  radiated  from  the  ball  and  con- 
centrated at  the  focus/'  by  the  reflectors. 

The  reflecting  power  of  substances  varies  not  only  with  the 
nature  of  their  surfaces,  but  also  with  the  material  of  which 
they  consist.  Polished  metallic  surfaces  are  the  best  reflectors 
of  heat,  and,  according  to  Leslie,  brass  and  silver  are  the  best 
reflecting  substances.  Non-metallic  bodies  have  very  low 
reflecting  powers ;  indeed,  many  of  them  entirely  absorb  all 
the  heat  which  impinges  upon  them. 

A  -body  absorbs  that  heat  which  it  does  not  reflect ;  hence 
the  absorptive  power  of  a  body  is  inversely  as  its  reflective 
power ;  and,  as  a  general  rule,  the  power  of  absorption  cor- 
responds with  the  power  of  radiation  :  thus,  for  example,  a 
surface  of  lampblack  has  no  reflective  power,  but  it  possesses 
the  highest  radiating  and  absorbing  power. 

Those  substances  which  allow  all  the  rays  of  heat  to  pass 
through  them  are  called  diather'manous  ;  and  those  substances 
which  retain  all  the  heat  they  receive  are  called  ather'manous. 

Gases,  such  as  the  air,  are  diathermanous ;  and  opaque  bodies,  such 
as  the  metals,  are  athermanous.  The  power  of  a  body  to  transmit  heat 
depends  upon  its  possessing  some  degree  of  transparency  ;  but  at  the  same 
time  it  is  remarkable  that  the  capacity  of  liquids  and  solids  for  transmit- 
ting heat  is  not  always  in  proportion  to  their  transparency  or  capacity 
for  transmitting  light.  Rock  salt  is  the  most  diathermanous  of  all  solids, 
and  alum  is  th,e  least.  Of  all  liquids  water  is  the  least  diathermanous. 


Leslie's  Experiments  on  the  Radiating  Powers  of  different 
Surfaces. 

In  these  experiments  a  small  canis- 
ter of  tin  was  employed,  one  side  of 
which  he  polished,  the  second  he  made 
rough  by  scraping,  the  third  he  cov- 
ered with  glass,  and  the  fourth  he 
coated  with  lampblack.  He  then  filled 
the  canister  with  boiling  water,  and  pre- 
sented the  different  sides  in  succession  in  7^.  97. 


192  NATURAL    AND    EXrEIlUlLNTAL    PHILOSOPHY.  ^ 

front  of  a  concave  reflector  M,  in  the  focus  of  which  he  placed  a  ball 
ft  of  a  delicate  differential  thermometer,  as  shown  in  Fig.  97.  With 
this  apparatus  he  first  verified  the  law,  that,  other  things  being  the 
same,  the  amount  of  radiant  heat  is  proportional  to  the  difference  between 
the  temperature  of  the  water  and  the  temperature  of  the  air.  He  then 
showed  that  the  radiation  is  proportional  to  the  extent  of  the  radiating 
surface,  and  inversely  as  the  distance  of  the  radiating  surface  from  the 
reflector.  He  further  showed  that  the  polished  face  of  the  canister  radi- 
ated the  least,  and  that  covered  with  lampblack  radiated  the  most ;  and 
so  on  to  the  radiating  powers  of  the  other  surface. 


CONDUCTION    OF   HEAT. 

67.  It  has  already  been  shown  (see  page  180)  that  bodies  differ  very 
much  in  their  powers  of  conducting  heat.  The  following  simple  experi- 
ment shows,  in  a  marked  manner,  the  difference  in  the  conducting 
powers  of  rods  of  different  substances. 

Exp.  Take  two  equal  rods,  (say  of  iron  and  glass  ;")  tie  them  together 
at  one  end  with  a  piece  of  fine  wire,  and  attach  a  ball  of  wax  at  their 
other  extremities,  b  and  d,  as  shown  in 
Fig.  98  ;  apply  the  flame  of  a  spirit  lamp 
to  the  part  a  c,  where  the  rods  are  con- 
nected ;  then  the  wax  on  the  iron  rod  will 
be  completely  melted,  while  that  on  the 
glass  rod  will  remain  unchanged ;  thereby 
showing  that  iron  is  a  much  better  con-  Fig.  98. 

ductor  of  heat  than  glass. 

The  propagation  of  heat  in  liquids  varies  according  to  the 
part  at  which  the  heat  is  applied.  When  the  heat  is  applied 
at  the  surface,  the  conduction  goes  on  very  slowly  ;  in  this 
case  the  liquid  conducts  heat  in  the  usual  manner :  but  when 
the  heat  is  applied  to  the  lower  portion  of  the  liquid,  the 
heated  particles,  being  specifically  lighter  than  the  cold  parti- 
cles, rise  successively  to  the  surface,  while  the  cooler  particles 
at  the  surface  descend ;  and  thus  this  constant  current  diffuses 
the  heat  equally  throughout  the  whole  mass.  But  this  trans- 
mission of  heat  takes  place  independently  of  the  ordinary 
principle  of  conduction  ;  it  is,  in  fact,  an  operation  of  convey- 
ance, not  of  conduction  from  particle  to  particle. 


LIGHT   AND    HEAT. 


193 


The  following  experiments  sufficiently  establish  the  truth  of  these 
laws  :  — 

Exp.  1.  Partly  fill  a  flask  with  water,  and  throw  some  powdered 
amber  (or  any  substance  having  nearly  the  same  specific  gravity  as  water) 
into  it ;  heat  the  water  by  a  spirit  lamp  ;  the  currents  in  the  fluid  will 
be  apparent  from  the  ascent  of  the  particles  of  amber  up  the  middle  of 
the  water,  and  from  their  descent  at  the  sides  of  the  vessel,  as  shown  in 
Pig.  99. 


Fig.  99. 


Fig.  100. 


Exp.  2.  A  (in  Pig.  100)  represents  a  tin  vessel  full  of  water ;  a  and  b 
thermometers  —  one  placed  near  the  top  of  the  vessel,  the  other  near  the 
bottom. 

Ploat  a  metal  cup  c,  filled  with  spirits  of  wine,  on  the  surface  of  the 
water ;  set  fire  to  the  spirit,  and  the  heat  produced  will  very  soon  be 
shown  by  the  thermometer  a,  but  the  thermometer  b  will  remain  for  a 
long  time  without  showing  any  indications  of  heat. 

Remove  the  cup  c,  and  place  the  vessel  on  a  lump  of  ice ;  the  lower 
thermometer  b  will  now  in  a  short  time  indicate  a  reduction  of  tempera- 
ture, while  the  upper  thermometer  a  will  remain  unchanged. 

These  experiments  show  that  water  is  a  bad  conductor  of  heat. 

Place  the  vessel  on  a  hot  plate  or  brick  ;  the  whole  of  the  water  will 
become  rapidly  heated,  and  both  thermometers  will  indicate  the  same  or 
nearly  the  same  amount  of  heat. 

Place  a  piece  of  ice  upon  the  surface  of  the  water  ;  both  thermometers 
will  speedily  indicate  a  decrease  of  temperature.  The  particles  of  the 
water,  as  they  cool,  become  heavier,  and  descend  to  the  bottom,  while 
the  lighter  particles  at  the  bottom  rise  to  the  top,  and  so  on. 

But  the  water  at  the  bottom  cannot  be  cooled  in  this  way  below  39°, 
17 


194          NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

for  water  attains  its  maximum  density  before  it  reaches  the  freezing  point, 
that  is,  at  a  temperature  of  39°. 


Heat  of  the   Ocean. 

68.  In  the  temperate  and  torrid  zones  the  temperature  of 
the  ocean,  generally  speaking,  diminishes  as  the  depth  below 
the  surface  increases ;  but  the  reverse  of  this  takes  place  in 
the  frigid  zones,  for  when  the  water  at  the  surface  is  less  than 
32°,  the  lower  portions  cannot  have  a  lower  temperature  than 
39°,  for  this  is  the  temperature  corresponding  to  the  greatest 
specific  gravity  of  water.      Hence  it  is  that  ice  is  always 
formed  upon  the  surface  of  the  water,  and  not  at  the  bottom. 
Hence  fishes  are  enabled  to  live  in  our  northern  seas. 

If  the  ordinary  law  of  density,  as  depending  upon  temper- 
ature, had  existed  in  this  case,  the  whole  of  the  northern  seas 
would  have  been  converted  into  one  solid  mass  of  ice. 

Heat  of  the  Atmosphere. 

69.  Aeriform  bodies  resemble  liquids  in  their  laws  of  con- 
duction and  conveyance  of  heat.     Still  air  is  the  worst  con- 
ductor of  heat,  but  by  the  rapid  ascent  and  descent  of  its 
particles,  it  distributes  heat  even  more  rapidly  than  water. 
The  various  motions  in  the  atmosphere,  noticed  in  Pneu- 
matics, depend  upon  this  property. 

Queries. 

70.  The  student  will  now  be  able  to  answer  the  following 
queries  :  — 

1.  Whether  will  water  boil  sooner  in  earthen  ware  vessels  or  in  tin 
ones? 

2.  What  is  the  use  of  having  double  windows  in  a  house  ? 

3.  Why  do  brass  cannon  become  sooner  hot  than  iron  ones  ? 

4.  If  we  touch  a  piece  of  wood  and  a  piece  of  metal,  both  at  a  tem- 
perature higher  than  our  body,  the  metal  feels  hotter  than  the  wood. 
Why? 


LIGHT   AND    HEAT.  195 

5.  Thatched  roofs  are  cooler  in  summer  and  warmer  in  winter  than 
slate  roofs.     Why  ? 

6.  What  is  the  use  of  making  boilers  and  saucepans  with  wide  bot- 
toms ? 

7.  Why  are  woollen  shirts  warmer  than  linen  ones  ? 

8.  What  part  of  a  crowded  church  is  the  cooles-t  ? 

9.  Snow  is  a  bad  conductor  of  heat.    What  is  the  utility  of  this 
property  ? 

10.  Why  does  a  man  blow  his  hands  to  make  them  warm,  and  his 
soup  to  make  it  cool  ? 

11.  Why  are  people  liable  to  catch  cold  when  they  get  their  clothes 
damp  ? 

12.  Why  should  an  ink  bottle  have  a  small  mouth  ? 

Rate  of  Conduction  in  Bodies. 

71.  The  rate  at  which  heat  is  conducted  depends  upon  the 
difference  of  temperature  between  the  bodies,  or  parts  of  a 
body,  that  are  in  contact.     This  will  be  manifest  from  the  fol- 
lowing experiment :  — 

Exp.  Plunge  a  thermometer  into  hot  water ;  at  first  the  ascent  of  the 
mercury  in  the  tube  is  very  rapid,  but  as  the  temperature  of  the  mercury 
in  the  bulb  approaches  the  temperature  of  the  water,  the  ascent  of  the 
mercury  goes  on  more  and  more  slowly. 

Heat  is  capable  of  diffusing  itself  more  or  less  rapidly 
through  the  particles  of  all  bodies. 

Capacity  of  Bodies  for  Heat.  —  Specific  Heat  of  Bodies. 

72.  The  amount  of  free  caloric  in  two  different  quantities 
of  the  same  substance  at  the  same  temperature  is  proportional 
to  their  masses :  thus,  two  pints  of  hot  water  will  contain 
twice  the  quantity  of  free  caloric  that  one  pint  of  the  water  at 
the  same  temperature  does. 

If  two  equal  quantities  of  water,  or  any  other  liquid,  of  different  tem- 
peratures, be  mixed,  the  heat  of  the  mixture  will  be  the  mean  of  the 
two  temperatures.  For  example,  if  a  pint  of  water  at  60°  be  mixed 
with  another  pint  of  water  at  80°,  the  temperature  of  the  mixture  will 
be  equal  to  one  half  of  140°,  or  70°.  If  their  quantities  are  unequal, 
their  common  temperature  after  mixture  will  be  found  by  dividing  the 


196          NATURAL    AND    EXPERIMENTAL   PHILOSOPHY. 

sum  of  the  products  of  their  masses  into  their  temperatures  by  the 
sum  of  their  masses  ;  thus,  if  we  mix  two  pints  of  water  at  60°  with 
three  pints  at  100°,  the  temperature  of  the  mixture  will  be  equal  to 

=  84°.    These  results  may  be  readily  proved  by  ex- 


periment. 

Equal  weights  of  dissimilar  substances  —  say  water  and 
mercury  —  at  the  same  temperature  contain  unequal  quan- 
tities of  heat.  If  we  place  1  Ib.  of  water  and  1  Ib.  of  mer- 
cury on  a  hot  plate,  it  is  obvious  that  the  mercury  will  attain 
any  given  temperature  much  sooner  than  the  water.  The 
water  is  said  to  have  a  higher  capacity  for  heat  than  the  mer- 
cury, for  it  requires  a  larger  quantity  of  heat  to  raise  it  to  the 
same  temperature.  The  quantity  of  heat  required  to  raise 
equal  weights  of  bodies  1°  is  called  their  specific  heat.  In 
general,  the  capacity  of  bodies  for  heat  decreases  with  their 
density;  thus  mercury  has  a  less  capacity  for  heat  than 
water,  because  its  density  is  greater  ;  thus  the  thin  air  on  the 
tops  of  mountains  has  a  higher  capacity  for  heat  than  the 
dense  air  in  the  plains. 

Experiment.  —  Mix  1  Ib.  of  mercury  at  66°  with  1  Ib.  of  water  at  32°  ; 
then  the  common  temperature  of  the  mixture  will  be  found  to  be  33°. 

Here  the  mercury  loses  33°,  and  the  water  gains  1°  ;  that  is  to  say, 
the  33°  of  the  mercury  only  elevates  the  water  1°,  therefore  the  capa- 
city of  water  for  heat  is  33  times  that  of  mercury  ;  or,  if  we  call  the 
capacity  or  specific  heat  of  water  1,  then  the  capacity  or  specific  heat  of 
mercury  will  be  -fa  or  .0303. 

In  this  way  the  specific  heat  of  various  bodies  may  be  determined. 
But  the  capacity  of  different  substances  for  heat  has  been  more  accurately 
determined  by  observing  the  quantity  of  ice  which  the  body  is  capable  of 
thawing.  The  instrument  employed  for  this  purpose  has  been  called  a 
cdhrim'eter. 

A  change  of  volume  is  invariably  attended  with  a  change  of 
specific  heat.  —  When  the  bulk  of  a  body  is  reduced,  its  ca- 
pacity for  heat  is  also  reduced,  and  free  caloric  is  evolved. 
When  the  bulk  of  a  body  is  increased,  its  capacity  for  heat  is 
also  increased,  and  free  caloric  is  absorbed  ;  and  hence  the 
change  is  followed  by  a  reduction  of  temperature. 


LIGHT    AND    HEAT.  197 

Exp.  1.  The  sudden  condensation  of  air,  in  a  small  tube,  having  a 
solid  piston  fitted  to  it,  will  ignite  tinder.  A  little  instrument  of  this 
kind  is  sold  by  philosophical  instrument  dealers. 

Exp.  2.  Air  forcibly  expelled  from  the  mouth  feels  cold.  Here  the 
cold  is  due  to  the  sudden  expansion  of  the  air. 

Exp.  3.  Iron  when  hammered  becomes  hot.  Here  the  hammering 
brings  the  particles  of  the  iron  nearer  to  one  another. 

Exp.  4.  Pour  water  on  some  quick  lime  ;  the  water  becomes  incorpo- 
rated with  the  solid  substance  of  the  lime,  the  specific  heat  of  the  two 
substances  is  reduced,  and  a  powerful  heat  is  therefore  evolved.  (See 
also  Experiments  1,  2,  and  3,  pages  185-6.) 

Heat  changes  the  Form  of  Bodies. 

73.  It  has  been  shown  (see  page  183)  that  when  a  body 
changes  its  form,  either  a  certain  quantity  of  free  heat  is  ab- 
sorbed and  becomes  latent,  or  a  certain  quantity  of  latent  heat 
is  evolved  and  becomes  free.  Thus,  when  a  body  passes  from 
the  solid  to  the  liquid  state,  or  from  the  liquid  to  the  gaseous 
state,  a  certain  quantity  of  free  caloric  is  absorbed ;  and,  on 
the  contrary,  when  a  body  passes  from  the  gaseous  to  the 
liquid  state,  or  from  the  liquid  to  the  solid  state,  a  certain 
quantity  of  latent  heat  is  set  free :  the  former  changes  pro- 
duce cold,  the  latter  are  attended  by  the  evolution  of  heat. 

The  change  of  solids  to  liquids  is  called  fusion  or  melting. 
The  change  of  solids  or  liquids  to  the  vaporous  or  gaseous 
state  is  called  vaporization  when  it  takes  place  with  boilino- 
or  ebullition,  but  when  the  change  takes  place  at  ordinary 
temperatures  of  the  air,  it  is  called  evaporation. 

Exp.  1.  Heat  a  small  piece  of  brimstone  in  a  test  tube  ;  the  brimstone 
first  melts,  and  then  rises  in  the  form  of  vapor  ;  these  vapors  condense  on 
the  upper  portion  of  the  tube. 

Exp.  2.  Heat  a  very  small  piece  of  iodine 
in  a  flask ;  the  flask  becomes  filled  Avith  the 
beautiful  violet- colored  vapor  of  iodine. 

Exp.  3.  Boil  some  water  in  a  retort,  and 
receive  the  condensed  steam  in  another  glass 
vessel.    (See  Fig.  101.)   This  is  called  dis- 
tilled water,  and  the  operation  is  called  dis-  Fi9>  101. 
tillation. 

17* 


198 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Exp.  4.  Apply  the  heat  in  the  last  experiment  so  as  to  cause  the  steam 
to  blow  out  of  the  nozzle  of  the  retort ;  now  quickly  invert  the  retort 
with  its  nozzle  in  the  water  ;  a  sudden  and  violent  condensation  of  all 
the  steam  in  the  retort  will  take  place,  and  the  cool  water  will  rush  into 
the  retort  and  will  completely  fill  it. 

The  Latent  Heat  of  Steam. 

74.  Fig.  102  represents  an  apparatus  for  generating  and  condensing 
steam,  so  as  to  enable  us  to  ascertain  the  amount  of  its  latent  heat.  A 
is  a  copper  boiler ;  a  b  the  steam  pipe 
leading  into  a  vessel  b  containing  water ; 
the  steam,  as  it  enters  the  cold  water,  is 
instantly  condensed,  and  imparts  its  la- 
tent heat  to  the  water  in  the  vessel  until 
it  reaches  the  boiling  point.  Let  5£  oz. 
of  water  at  32°  be  placed  in  the  vessel  b, 
and  let  the  processes  of  condensation  go 
on  until  the  water  is  raised  to  the  tem- 
perature of  212°;  then  it  will  be  found 


Fig.  102. 


that  there  are  6^  oz.  of  water  in  the  vessel  b;  that  is  to  say,  the  con- 
densation of  1  oz.  of  steam  has  raised  5£  oz.  of  water  from  the  temper- 
ature of  32°  to  that  of  212°.  Hence  it  follows  that  the  latent  heat  of  a 
given  portion  of  steam  is  capable  of  raising  5£  times  its  weight  of  water 
180°,  and  therefore  it  will  raise  an  equal  weight  of  water  5£  times  180°, 
or  990°,  or,  in  round  numbers,  1000°.  This  number,  therefore,  repre- 
sents the  latent  heat  of  steam. 


Expansive  Force  of  Steam. 

75.  When  steam  is  generated  in  a  close  vessel,  like  the 
boiler  of  a  steam  engine,  as  the  temperature  of  the  water  is 
raised,  so  the  density  and  pressure  of  the  steam  is  raised  ac- 
cordingly. 

Fig.  103  represents  an  apparatus  for  ascertaining  the  law  of  relation 
between  the  temperature  and  pressure  of  steam.  B  is  the  boiler  partly 
filled  with  water  ;  L  the  heat  applied  to  it ;  A  B  a  barometer  tube,  open 
at  both  ends,  dipping  into  a  portion  of  mercury  at  the  lower  part  of  the 
boiler  ;  T  a  thermometer  with  its  bulb  inserted  in  the  steam  ;  C  a  stop 
cock,  which  may  be  closed  or  opened  at  pleasure.  Now,  when  the  tem- 
perature of  the  steam  is  raised  above  the  boiling  point,  (212°,)  the  mer- 
cury rises  in  the  barometer  tube  A,  and  the  height  of  the  mercury  and 


LIGHT    AND    HEAT. 


199 


the  temperature,  as  indicated  by  the  thermom- 
eter, being  observed  at  the  same  instant,  gives 
us  the  means  of  determining  the  relation  be- 
tween the  pressure  and  temperature  of  the 
steam.  When  the  steam  issues  from  the  cock 
C,  the  mercury  in  the  tube  A  B  is  at  the  same 
level  as  the  mercury  in  the  boiler,  and  then 
the  pressure  of  the  steam  is  the  same  as  that  of 
the  atmosphere,  \vhich  we  estimate  at  a  column 
of  30  inches  of  mercury  ;  when  the  stop  cock 
is  closed,  and  the  mercury  in  the  tube  A  B 
rises,  we  must  add  this  column  of  mercury  to 
the  30  inches  for  the  total  column,  balancing  the 
pressure  of  the  steam  in  the  boiler.  Thus,  for 
example,  when  the  temperature  is  232°,  the 
mercury  iri  the  tube  A  will  stand  at  the 
height  of  15  inches  above  the  level  of  the 
mercury  in  the  boiler,  that  is  to  say,  the  pres- 
sure of  the  steam  in  the  boiler  will  be  meas- 
ured by  a  column  of  mercury  equal  to  45 
inches,  or,  in  other  words,  the  pressure  of  the 
steam  at  232°  will  be  1^  times  the  pressure 
of  the  atmosphere,  or  equal  to  about  22  £  Ibs. 
per  square  inch.  Experimental  tables  have 
been  constructed,  giving  the  relation  of  the 
temperature  and  pressure  of  steam. 

"Various  simple  pieces  of  apparatus  have 
been  constructed  to  illustrate  the  expansive 
force  of  steam  generated  under  high  pressure. 


Fig.  103. 


Fig.  104  shows  how  the  expansive  force  of  a  jet  of  steam  issuing  from 
the  pipe  a  b,  and  impinging  upon  the  vanes  of  a  wheel  "W,  is  capable  of 
imparting  a  rotating  motion  to  the  wheel. 


Fig.  104. 


200 


NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 


Fig.  105. 


Fig.  106. 


Fig.  105  shows  how  the  reaction  of  the  steam,  issuing  from  the  nozzles 
b  b  b  b,  gives  a  rotatory  motion  to  the  globe  A. 

Fig.  106  shows  how  a  jet  of  steam,  projected  on  the  flame  b,  may  be 
used  as  a  blast  for  a  blowpipe. 

Dalton  used  the  Torricellian  tube  to  estimate  the  expansive  force  of 
steam,  at  temperatures  below  the  boiling  point  of 
water.  He  filled  the  Torricellian  tube  b  d  with 
mercury,  and  inverted  it  in  the  mercury  contained 
in  the  vessel  A  ;  he  then  introduced  into  the  tube 
a  small  quantity  of  the  liquid  whose  vapor  he 
wished  to  examine.  This  liquid  rises  to  the  top  of 
the  mercury,  and  occupies  a  portion  of  the  vacuum 
space  ;  it  there  gives  off  its  vapor,  which  causes  the 
mercury  to  sink  in  the  tube,  and  to  stand  at  a  height 
m  corresponding  to  the  elasticity  of  this  vapor, 
which  is  dependent  upon  its  heat.  This  reduction 
of  the  column  of  mercury  gives  the  column  of  mer- 
cury due  to  the  pressure  of  the  vapor  at  the  partic- 
ular temperature.  In  order  to  vary  the  tempera- 
ture  at  pleasure,  he  surrounded  the  upper  part  of 
the  Torricellian  tube  with  a  wide  glass  tube  c,  and 
filled  it  with  water,  in  which  he  placed  a  thermom- 
eter. He  then  heated  this  water,  which  caused  the 
mercury  to  descend  in  the  tube,  and  occupy  a  posi- 
tion corresponding  to  the  elasticity  of  the  vapor  in 
the  upper  portion  :  by  noting  the  temperature  and  J? 
the  height  of  the  mercury  at  the  same  instant,  he 
was  enabled  to  give  the  relation  between  the  temperature  of  the  vapor 
and  its  elastic  pressure. 


LIGHT    AND    HEAT. 


201 


EVAPORATION. 

76.  The  leading  laws  and  effects  of  evaporation  have  been 
explained.  The  following  experiments  and  expositions  are 
intended  to  place  the  subject  in  a  more  prominent  and  scien- 
tific point  of  light. 

Exp.  1.    Water  frozen  by  evaporation.  —  a  a  is  a  glass  vessel  contain- 
ing strong  sulphuric  acid,  over  which  is 
placed  a  tripod  stand,  supporting  a  po- 
rous cup  c  c  of  earthen  ware,  filled  with 
cold  water  :  the  whole  is  placed  under  a 
glass  bell  d  d,  on  the  plate  of  a  good  air 
pump.     When  the  air  is  exhausted  from 
the  receiver,  the  water  in  the  cup  be- 
comes   gradually    converted     into     ice. 
Here  the  exhaustion  of  the  air  causes  the 
evaporation  of  the  water  to  go  on  with  increased  rapid- 
ity; at  the  same  time,  the  sulphuric  acid  absorbs  this 
vapor  as  it  is  being  formed :  the  rapid  evaporation  from 
the  surface  of  the  water  produces  a  cold  sufficient  to 
freeze  it. 

Exp.  2.  Mercury  frozen  by  evaporation.  —  Q  is  a  mer-  g 
curial  thermometer,  and  W  a  spirit-of-wine  one,  fixed 
side  by  side ;  a  a  is  a  glass  vessel  placed  about  three 
inches  below  the  balls  of  the  thermometers,  which  are 
wrapped  with  cotton,  saturated  with  rectified  sulphuric 
ether ;  the  whole  is  placed  under  a  receiver  g  g,  on  the 
plate  of  an  air  pump.  When  the  air  is  exhausted  from 
the  receiver,  the  cold  produced  by  the  rapid  evaporation 


Fig.  109. 


Fig.  110. 


of  the  ether  will  cause  the  mercury  to  become  solid  ;  the  temperature  at 
which  this  takes  place,  as  indicated  by  the  spirit-of-wine  thermometer, 
is  about  40°  below  zero. 

The  cryophorus  was  invented  by  Wollaston,  for  freezing  water  by  its 
own  evaporation :  it  consists  of  a  glass  tube  A  B  (see  Fig.  Ill)  termi- 
nated with  two  bulbs  C  and  D,  on 
short  branches  bent  at  right  angles  to 
it.  A  portion  of  water  is  introduced 
through  a  little  tube  O  at  the  bottom  c  I 
of  the  bulb  D.  '  This  water  is  then 
boiled  in  C  until  all  the  air  is  blown 
out,  and  the  whole  interior  space  is  filled  with  steam ;  the  tube  O  is 
then  closed  by  melting  its  fine  extremity  in  the  flame  of  a  blowpipe. 
When  the  instrument  has  become  cool,  the  enclosed  water  is  then  sur- 


Fiff.  111. 


202 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


rounded  by  its  own  vapor  raised  in  a  vacuum.  Having  described  the 
instrument,  let  us  now  see  how  it  is  used.  Plunge  the  empty  bulb 
D  into  a  freezing  mixture  of  snow  and  salt,  and  the  water  in  the 
other  bulb  will  speedily  be  turned  into  ice.  Here  the  cold  produced 
by  the  freezing  mixture  is  constantly  condensing  the  aqueous  vapor  in 
the  bulb  D  ;  and  as  there  is  no  air  in  the  tube,  more 
and  more  vapor  is  continually  rising  from  the  water 
in  the  other  bulb,  and  this  goes  on  until  so  much 
heat  is  abstracted  from  the  water  by  the  evaporation 
that  it  freezes. 

Fig.  112  represents  another  form  of  this  instru- 
ment, where  sulphuric  ether  is  substituted  for  the 
freezing  mixture,  d  is  the  bulb  containing  the  water 
which  is  to  be  frozen;  c  the  empty  bulb  wrapped 
round  with  some  cotton  moistened  with  ether  ;  to 
hasten  the  evaporation  of  the  ether,  air  is  blown 
from  a  pair  of  bellows  upon  the  bulb  c.  The  experi- 
ment may  be  performed  more  quickly  by  enclosing  c 
in  the  exhausted  receiver  a  b  a  of  an  air  pump. 


Moisture  in  the  Air.  —  Hygrometers. 

77.   The  instruments  used  to  measure  the  moisture  of  the   air  are 
called  hygrometers. 

Saussures  hair  hygrometer.  —  The  mode  of  action  of  this 
instrument  depends  on  the  fact,  that  substances  like  hair 
readily  imbibe  moisture  from  a  damp  atmosphere,  and,  on 
doing  so,  swell  out  in  thickness,  but  contract  in  length. 
This  instrument  is  represented  in  Fig.  113.     A  B  is  a  hu- 
man hair,  about  one  foot  long,  (freed  from  all  grease  by 
boiling  it  in  a  weak  solution  of  soda  or  potassa  ;)  one  end 
of  it  is  fastened  to  a  hook  at  B,  and  the  other  is  passed  over 
a  fixed  pulley  P,  and  is  strained  tight  by  means  of  a  small 
weight  "W.     The   contraction   and  expansion  of  this  hair 
give  motion  to  an  index  pointer  C,  which  moves  over  the     Fig.  113. 
face,  of  the  graduated  arc  a.     The  two  extreme  points  of 
this  scale  are,  where  the  index  pointer  stands  in  a  perfectly  dry  atmos- 
phere, and  where  it  stands  in  an  atmosphere  saturated  with  moisture. 
The  former  point  is  marked  0,  or  zero,  and  the  latter  is  usually  marked 
100  ;  and  the  intervening  arc  is  divided  into  100  equal  parts,  each  part 
being  called  a  degree.     The  zero  point  is  obtained  by  enclosing  the  in- 
strument in  a  glass  bell,  from  which  the  aqueous  vapor  is  withdrawn  by 
means  of  dry  chloride  of  calcium,  or  strong  sulphuric  acid :  the  point 


LIGHT   AND    HEAT.  203 

of  greatest  humidity  is  determined  by  placing  the  instrument  in  a  glass 
bell  standing  over  water.  It  must  be  observed,  however,  that  the  degrees 
of  humidity  shown  by  this  instrument  are  not  exactly  in  proportion  to 
the  humidity  existing  in  the  air :  indeed,  it  is  exceedingly  difficult  to 
make  a  perfect  hygrometrical  instrument. 

The  experiment  explained  at  page  184  gives  a  simple  and  tolerably 
accurate  method  of  determining  the  dew  point  of  the  atmosphere  at  any 
time. 

The  mercurial  hygrometer.  —  A  good  instrument  of  this  kind  is  rep- 
resented in  Fig.  114.     A  and  B  are  two  ordinary  mercu- 
rial thermometers  placed  by  the  side  of  each  other  ;  the 
bulb  A  is  surrounded  with  muslin,  which  is  kept  in  a 
damp  state  by  means  of  a  cotton  thread  attached  to  it, 
the  other  end  of  the  thread  being  placed  in  a  cup  W  of 
distilled  water ;  the  other  bulb  B  is  kept  dry.     Now,  as 
the  water   evaporates  from  the  muslin,  the  mercury  in 
the  thermometer  A  falls  ;  and  the  drier  the  surrounding 
air  at  the  time,  the  more  rapid  is  the  descent  of  the  mer- 
cury ;  and  when  the  air  about  the  bulb  A  becomes  satu- 
rated with  moisture,  the  mercury  in  the  tube  becomes 
stationary,  and  the  point  at  which  it  stands  will  be  the 
dew  point  of  the  air  at  that  time.     The  greater  the  dif-       Fig.  114. 
ference  between  the  two  thermometers,  the  greater  will 
be  the  dryness  of  the  air.     When  the  damp  thermometer  A  indicates  no 
decrease  of  temperature,  then  the  surrounding  air  at  that  moment  will 
be  saturated  with  moisture. 

Certain  Meteorological  Phenomena.  —  Dew.  —  Clouds.  — 
Rain.  —  Snow. 

78.  When  the  surface  of  the  earth  has  become  heated,  it 
imparts  its  heat  by  radiation  to  the  surrrounding  air ;  during 
the  day,  therefore,  the  lower  strata  of  the  atmosphere  are 
warmer  than  the  upper  But  a  change  takes  place  after  sun- 
set. The  surface  of  the  earth  still  goes  on  radiating  its  heat ; 
but  it  now  receives  no  heat  in  exchange,  so  that  its  tempera- 
ture at  length  falls  below  that  of  the  air.  Now,  the  air  imme- 
diately in  contact  with  the  cold  surface  of  the  earth  becomes 
cooled  down,  and,  if  the  temperature  falls  to  the  dew  point  of 
the  air,  then  the  vapors  in  it  are  condensed  on  the  surface  of 
the  plants,  or  the  soil,  in  the  form  of  small  drops  or  dew,  just 


204         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

in  the  same  way  as  the  cold  tumbler  becomes  covered  with 
moisture  in  a  warm  room.  If  the  temperature  of  the  earth's 
surface  sinks  to  the  freezing  point,  the  moisture  is  deposited  in 
the  form  of  hoar  frost. 

The  surface  of  the  earth  radiates  most  heat  when  the  sky- 
is  clear  and  serene ;  but  it  is  especially  obstructed  by  clouds, 
which,  like  a  screen,  radiate  back  to  the  earth  the  heat  which 
they  receive  from  it.  Hence  it  is,  that  clear,  cloudless  nights 
are  most  favorable  for  the  formation  of  dew. 

To  arrest  the  radiation  of  heat  from  the  earth,  the  gardener 
covers  his  tender  plants  with  matting  or  straw. 

Winds  and  mountains  are  the  great  causes  of  rain.  When 
a  warm  air,  nearly  saturated  with  moisture,  is  mixed  with 
cold  air,  moisture  is  always  precipitated,  and  becomes  visible 
to  us,  assuming  either  the  form  of  clouds  or  rain. 

Suppose  a  warm  air  to  be  driven  by  a  wind  towards  a  mountain  M  ; 
when  the  air  strikes  the  sloping  side  of  the  mountain,  it  rises,  on  the 
principle  of  the  resolution  of  motion,  and  it  is  carried  over  the  mountain 


Fig.  115. 

top ;  by  this  means  the  air  is  carried  from  a  warm  to  a  cold  region ;  its 
moisture,  therefore,  is  precipitated,  and  assumes  either  the  form  of 
clouds  or  rain,  or,  it  may  be,  when  the  cold  is  very  great,  that  of  snow 
or  hail.  Hence  rivers  take  their  rise  in  mountain  ranges. 

Mountains  also  frequently  attract  the  clouds  towards  them,  and  thus 
cause  rain  to  fall. 

The  two  great  processes  of  evaporation  and  condensation 
are  the  means  whereby  the  vegetation  of  the  earth  is  contin- 


LIGHT    AND    HEAT.  20<3 

ually  supplied  with  moisture  from  the  great  reservoir  of 
waters  —  the  ocean  —  which  covers  the  larger  portion  of  the 
globe. 

Philosophers  have  found  it  difficult  to  account  for  the  suspension  of 
the  particles  of  moisture  in  the  clouds.  It  is  generally  believed  that  the 
moisture  in  the  clouds  assumes  the  form  of  vesicles  of  watery  vapor,  or 
little  buoyant  air  bubbles,  which,  being  in  the  same  electrical  state  as 
the  stratum  of  air  immediately  below  them,  not  only  repel  one  another, 
but  are,  at  the  same  time,  repelled  by  the  air  beneath ;  and  thus  they  are 
supported  in  opposition  to  the  force  of  gravity.  Query.  May  not  these 
vesicles  be  supported  in  the  same  way  as  a  little  sewing  needle  is  sup- 
ported upon  the  surface  of  water  ? 
18 


ELECTRICITY. 


PRELIMINARY  VIEWS    AND  EXPERIMENTS. 

1.  WHAT  is  electricity  ?  It  is  a  subtile  fluid  which  per- 
vades all  nature,  and  which  becomes  known  to  us  by  its  pecu- 
liar properties,  or  by  the  way  in  which  it  affects  our  senses. 
Lightning  is  electricity ;  in  the  thunder  storm  nature  gener- 
ates the  electric  fluid  on  a  mighty  scale.  Electricity  is  most 
easily  generated  by  friction ;  or,  to  speak  more  definitely,  it 
is  rendered  apparent  to  our  senses  when  certain  bodies  are 
rubbed  against  each  other.  Electricity  appears  to  exist  in 
all  bodies,  in  a  latent  or  hidden  state ;  but  friction-  and  other 
causes  disturb  this  state  of  quiescence  or  inactivity.  There 
are  various  means  of  generating  electricity  besides  friction,  — 
for  instance,  heat,  chemical  action,  or  pressure  will  generate  it, 
—  but  we  purpose  first  to  show  its  various  properties  when  it 
is  generated  by  friction. 

EASY    COURSE    OF   EXPERIMENTS,    WITH    SIMPLE    PRINCIPLES 
DERIVED    FROM    THEM. 

2.  The  following  electrical  experiments  may  all  be  performed  by  any 
intelligent  person,  with  no  other  apparatus  than  what  may  be  obtained 
in  any  ordinary  dwelling  house.  All  the  experiments  here  described 
have  been  repeatedly  performed  by  the  author  with  invariable  success. 
Many  of  them,  he  believes,  are  new  and  simple,  and  highly  calculated 
to  interest  young  persons,  from  the  very  fact  that  they  have  it  in  their 
power  to  repeat  the  experiments  at  any  time  they  may  wish  to  do  so. 

Bodies  which  are  electrified,  or  which  contain  free  electri- 
city, attract  and  repel  light  substances ;  and  when  tire  elec- 
•tricity  is  generated  in  a  sufficient  quantity,  luminous  sparks, 

(206) 


ELECTRICITY. 


207 


accompanied  by  a  sharp,  cracking  noise,  pass  from  the  electri- 
fied body  to  any  body  which  is  not  electrified. 

These  fundamental  facts  of  electricity  are  illustrated  by  the  following 
experiments  :  — 

Exp.  1.  Rub  a  stick  of  stealing  wax,  or  a  dry 
glass  tube,  with  a  warm  piece  of  flannel  or  silk : 
electricity  is  generated.  Hold  the  excited  stick 
of  sealing  wax  over  some  cuttings  of  light  paper, 
or  any  other  light  substances :  the  bits  of  paper 
will  be  attracted  by  the  sealing  wax,  and  will 
sometimes  fly  and  dance  up  and  down. 

Bring  the  excited  sealing  wax  before  your 
eyes :  a  sensation  is  felt  as  if  spiders'  webs  were 
drawn  across  your  face. 

Bring  the  sealing  wax  under  your  nose  :  you 
feel  a  faint  smell  like  phosphorus. 

Suspend  a  feather  or  a  little  cork  ball  by  a  silk 
thread,  as  shown  in  Fig.  2 ;  bring  the  excited 
sealing  wax  near  the  little  ball :  it  is  first  at- 
tracted, and  then  it  is  repelled. 

Those  substances  which  readily  yield  electricity  by  friction 
have  been  called  electrics.  But  it  has  recently  been  found 
that  all  substances  possess  this  property  in  a  greater  or  less 
degree. 

Exp.  2.  Suspend  two  feathers  (or  two  light 
cork  balls)  by  silk  threads,  as  shown  in  Fig.  3  ; 
bring  the  excited  sealing  wax  near  the  feathers  : 
they  are  first  attracted  to  the  sealing  wax,  and 
then  they  are  repelled  from  it ;  and  they  will 
finally  be  found  to  diverge  or  fly  from  each  other, 
as  shown  in  the  figure. 

Here  it  will  be  observed  that  the  electrified 
body  first  attracts  the  feathers ;  and  then,  when 
they  become  electrified,  in  the  same  way  as  the 
sealing  wax,  they  are  repelled  by  it,  and  mu- 
tually repel  each  other. 

Exp.  3.  Bring  the  excited  stick  of  stealing  wax  near  a  light,  downy 
feather,  floating  in  the  air  :  the  feather  will  first  be  attracted  to  the  seal- 
ing wax,  and  then  repelled  from  it.  As  the  sealing  wax  is  moved 


Fig.  3. 


208 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  4. 


towards  the  feather,  it  will  continue 
to  fly  away.  In  this  way  the  feather 
may  be  driven  about  the  room.  If 
the  feather  should  touch  any  body  not 
electrified,  it  will  fly  back  to  the  seal- 
ing wax  again.  Or  if  an  excited  glass 
tube  be  brought  near  the  feather,  it 
will  be  attracted. 

Here  the  excited  sealing  wax  first 
attracts  the  feather ;  and  then,  when  the  feather  becomes  electrified  in 
the  same  way  as  the  sealing  wax,  it  is  repelled. 

Exp.  4.  Take  up  a  black  cat,  which  has  been  lying  before  the  fire ; 
hold  it  by  the  throat  with  one  hand,  and  with  the  other  hand  rub  it 
smartly  along  the  back  :  electricity  will  be  generated  ;  the  hair  will  be- 
come so  excited  and  charged  with  the  electrical  fluid  that  a  faint  shock 
may  sometimes  be  felt,  and  a  succession  of  sparks  may  be  seen,  if  the 
experiment  is  performed  in  the  dark. 

Exp.  5.  Take  two  strips  of  brown  paper, 
about  9  inches  long  and  2  inches  wide; 
warm  them,  and  hold  them  by  the  finger 
and  thumb  of  the  left  hand;  rub  them 
briskly,  by  inserting  the  fore  finger  of  the 
right  hand  between  them,  and  then  draw- 
ing it  sharply  from  end  to  end  :  the  strips 
of  paper  will  be  powerfully  electrified,  and 
will  diverge  from  each  other,  as  shown  in 
Fig.  5.  They  repel  each  other  because  they 
are  electrified  in  the  same  way. 

Bring  the  hand  between  them  when  thus  repelled,  and  they  will  both 
be  attracted  by  the  hand. 

Exp.  6.  Lay  the  two  strips  of  brown  paper,  the  one  over  the  other, 
on  a  smooth  table ;  rub  them  with  the  hand,  or,  what  is  still  better,  draw 
the  edge  of  an  ivory  rule  or  scale  over  them  for  a  few  times  ;  lift  them 
from  the  table,  and  then  separate  them  from  each  other :  they  will  attract 
each  other  very  powerfully. 

In  the  last  experiment,  they  repelled  each  other  because  they  were 
electrified  in  the  same  way ;  but  here  they  attract  each  other,  because 
they  are  electrified  in  different  ways.  It  will  be  afterwards  shown  that, 
whilst  the  bottom  piece  of  paper  is  positively  electrified,  the  top  piece  is 
negatively  electrified. 

The  two  foregoing  experiments  may  be  performed  with  silk  ribbons,  or 
with  strips  of  thin  sheet  gutta  percha. 

In  the  place  of  the  hand,  an  old  fur  cuff,  or  a  hare's  skin,  or  Indian 


Fig.  5. 


ELECTRICITY. 


209 


rubber,  or  a  piece  of  warm  flannel;  or  an  ivory  scale,  may  be  employed 
as  the  rubber. 

Exp.  7.  Take  two  pieces  of  lump  sugar,  and  rub  them  together  in  the 
dark  :  they  will  appear  covered  with  a  beautiful  lambent  flame  of  elec- 
tric light. 

Exp.  8.  Take  a  piece  of  stout  common  brown  paper  (or  a  sheet  of 
thin  gutta  percha)  about  a  foot  long  and  nine  inches  broad ;  hold  it 
before  the  fire  until  it  is  quite  hot,*  lay  it  upon  the  table,  and  rub  it 
briskly  for  a  few  times  with  the  palm  of  the  hand  :  the  paper  will  be- 
come powerfully  electrified. 

(1.)  Lift  the  paper  by  one  corner  from  the  table,  and  it  will  be  found 
that  some  force  is  required  to  separate  the  paper  from  the  table. 

(2.)  Hold  the  electrified  paper  as  in  Fig.  6;   bring  the  extended 


Fig.  6. 

fingers  of  the  right  hand  near  to  the  surface  of  the  paper :  it  will  be 
attracted  to  the  hand,  and  electric  sparks,  giving  a  snapping  sound,  will 
pass  from  it  to  the  fingers. 

(3.)  Hold  some  feathers,  suspended  by  silk  thread,  near  to  the  excited 
paper,  as  shown  in  Fig.  7  ;  the  feathers  will  be  powerfully  attracted. 

(4.)  Hold  the  excited  paper,  or  the  excited  sheet  of  gutta  percha,  as 
the  case  may  be,  over  the  head  of  a  person  having  dry  hair,  as  shown  in 
Fig.  8  ;  the  hair  will  be  powerfully  attracted  by  the  paper,  and  each  par- 
ticular hair  will  appear  as  if  standing  on  end. 

(5.)  Hold  the  excited  paper  over  the  face ;  a  sensation  like  that  pro- 
duced by  cobwebs  spread  over  the  face  will  immediately  be  felt. 

(6.)   Perform  experiments  1,  2,  and  3  with  the  electrified  paper. 

Exp.  9.  Take  two  pieces  of  stout  brown  paper,  of  the  same  size  as 
that  described  in  Exp.  8  ;  after  heating  them,  lay  the  one  upon  the  other, 

*  Gutta  percha  should  never  be  heated. 
18* 


210 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  7. 


and  rub  the  upper  surface  with  the  palm  of  the  hand ;  tear  the  two  sheets 
of  paper  from  the  table  ;  they  will  adhere  firmly  together.  A  crackling 
sound  will  be  heard,  upon  separating  them  from  each  other ;  and  upon 


Fig.  8. 

being  brought  near  to  each  other,  they  will  mutually  attract  each  other, 
and  will  again  adhere  to  each  other.  If  they  are  separated  from  each 
other  in  the  dark,  an  electric  flash  will  be  distinctly  observed. 

Exp.  10.  Hold  the  excited  paper  near  to  the  wall  of  the  room  :  the 
paper  will  fly  to  the  wall,  and  will  remain  there  some  minutes  without 
falling. 

Exp.  11.  To  obtain  a  scries  of  electric  sparks.  —  Take  a  small  tea 
tray,  and  place  it  upon  a  dry  tumbler  glass,  as  shown  in  Fig.  9  ;  place 
the  excited  sheet  of  paper  or  gutta  percha,  described  in  Exp.  8  or  9, 


ELECTRICITY. 


211 


upon  the  tea  tray  ;  bring  the  knuckle  near  to  the  tea  tray,  and  an  elec- 
tric spark  will  be  received ;  quickly  withdraw  the  paper,  and  again  apply 
the  knuckle,  and  another  spark  will  be  received  ;  replace  the  paper,  and 
then  apply  the  knuckle,  and  another  spark  will  be  perceived ;  and  so  on 
for  at  least  a  dozen  times. 


Fig.  10. 


Fig.  9. 

Exp.  12.  Take  a  small  splinter  of  wood,  about  9  inches  long ;  fix 
corks  to  its  extremities ;  suspend  it  from  its  middle  by  a  silk  thread  ; 
bring  the  excited  stick  of  sealing  wax,  or  brown  paper,  near  to  one  of 
the  corks  :  it  will  be  attracted,  and,  by  moving  the  electrified  body  in  a 
circle,  the  cork,  being  constantly  attracted,  will  appear  to  revolve  on  the 
thread  as  an  axis. 

Exp.  13.  Make  a  notch  in  the  mid- 
dle of  the  rod,  described  in  the  last 
experiment,  and  balance  it  on  the 
edge  of  a  dinner  knife  C,  as  shown  in 
Fig.  10.  (The  balance  of  the  rod  is 
easily  adjusted  by  pushing  either  the 
one  cork  or  the  other  nearer  to  the 
centre  of  the  splinter.)  Bring  the 

excited  brown  paper  over  the  cork  A,  and  it  will  be  attracted ;  now 
place  the  excited  brown  paper  over  the  cork  B,  and  it  will,  in  its  turn, 
be  attracted,  and  so  on  ;  thereby  giving  an  oscillating  motion  to  the  rod 
A  B  on  the  edge  C.  This  experiment  exhibits  electrical  attraction  in  a 
striking  manner,  being  conducted  on  a  large  scale. 

Exp.  14.  To  make  two  forks  revolve  by  electrical 
attraction.  —  Stick  two  small  forks  A  and  B  into  a 
cork  C,  as  shown  in  Fig.  11 ;  stick  a  sewing  nee- 
dle, with  its  point  outwards,  into  the  cork  ;  poise 
the  whole  on  the  point  of  the  needle  standing  on 
the  top  of  a  wine  glass  G  ;  bring  the  excited  seal- 
ing wax  or  brown  paper  near  one  of  the  forks,  and 
make  it  revolve,  as  in  Exp.  12. 

Exp.  15.  Blow  out  a  lighted  candle  having  a 
long  snuff;  bring  an  excited  rod  of  sealing  wax 


Fig.  11. 


I    UNIVERSITY 

Vc,     OF 


212  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

near  to  the  wick,  as  shown  in  Fig.  12 ;  the 
smoke  is  attracted  by  the  sealing  wax,  and 
sparks  of  fire  appear  to  fly  towards  it. 

Exp'.  16.  Support  a  warm  pane  of  glass 
upon  two  books,  one  at  each  end ;  place 
some  dry  bran,  or  cuttings  of  fine  paper,  or 
light  pith  or  cork  balls,  beneath  the  glass,  and 
briskly  rub  the  upper  side  v.ith  a  warm  piece 
of  flannel  or  black  silk  :  the  bran  will  dance 
up  and  down  with  great  rapidity. 

This  experiment  was  first  made  by  Newton.  It  was  important  at  the 
time  of  its  discovery,  inasmuch  as  it  showed,  what  was  not  known  be- 
fore, that  an  electrical  body  became  electrified  on  the  side  contrary  to 
that  which  was  excited. 

CONDUCTORS    AND    NON-CONDUCTORS    OF    ELECTRICITY. 
INSULATION. 

3.  The  metal  tea  tray  of  Exp.  11  was  placed  upon  a  glass 
tumbler,  because  glass  is  a  non-conductor  of  electricity ;  and, 
in  like  manner,  the  feathers  of  Exp.  2  were  suspended  by  silk 
threads,  because  dry  silk  is  a  non-conductor  of  electricity. 
If  the  feathers  had  been  suspended  by  a  metallic  wire,  in  the 
place  of  silk,  they  would  not  have  diverged  from  each  other 
in  the  manner  described,  for  metals  conduct  the  electric  fluid. 

The  electric  fluid  does  not  diffuse  itself  over  the  surface  of 
a  non-conductor,  but  remains  confined  strictly  to  that  portion 
of  the  surface  which  first  received  it ;  thus,  when  one  end  of 
a  stick  of  sealing  wax  is  rubbed,  that  extremity  becomes  higlily 
electrified,  whilst  the  other  extremity  remains  in  its  natural 
state.  On  the  contrary,  conductors  freely  convey  the  electric 
fluid  from  one  part  of  their  surface  to  another;  and  thus  the 
electric  fluid  instantaneously  diffuses  itself  uniformly  over  the 
whole  surface  of  the  conductor,  just  as  water  would  spread 
itself  over  a  level  surface.  All  metallic  bodies  are  excellent 
conductors ;  and  water,  wood,  &c.,  as  well  as  all  substances  in 
a  damp  state,  readily  conduct  electricity.  The  earth  is  the 
great  reservoir  and  conductor  of  electricity.  "When  any  elec- 
trified body  is  suspended  from  or  supported  by  a  non-con- 


ELECTRICITY. 


213 


ductor,  the  body  is  said  to  be  insulated.  All  non-conductors, 
therefore,  are  called  insulators.  Glass  rods,  silk  threads, 
sealing  wax,  and  fine  threads  of  sealing  wax,  are  the  insula- 
tors most  commonly  used  in  performing  electrical  experiments. 
All  these  substances  become  conductors  when  they  are  in  a 
damp  state ;  hence  the  necessity  of  having  all  our  insulators 
perfectly  dry  and  warm.  Sealing  wax  is  the  best  of  all  insu- 
lators, because  it  does  not  readily  become  covered  with  mois- 
ture. For  conducting  delicate  experiments,  there  is  no  insu- 
lator to  be  compared  with  a  fine  thread  of  sealing  wax  or 
gum  lac. 

Bodies  differ  very  much,  as  well  in  their  conducting  as  in 
their  insulating  powers.  Of  all  bodies  metals  are  the  best 
conductors,  and  resinous  bodies  are  the  best  insulators  or  non- 
conductors. The  bodies  in  the  following  list  possess  these 
powers,  in  the  order  in  which  they  are  named. 


Classification    of    Conductors     ac- 
cording to  their  conducting  power. 

1.  All  the  metals. 

2.  Charcoal. 

3.  Plumbago. 

4.  Acids. 

5.  Metallic  ores. 

6.  Animal  fluids. 

7 .  Water,  and  all  damp  substances. 

8.  Ice  above  13°  Fahrenheit. 

9.  Snow. 

10.  Living  animals  and  vegetables. 

11.  Flame,  smoke,  and  steam. 

12.  Soluble  salts. 

13.  Rarefied  air. 

14.  Vapors  of  ether  and  alcohol. 

15.  Damp  earth  and  stones. 

16.  Powdered  glass. 

17.  Flowers  of  sulphur. 


Classification  of  Insulators  accord- 
ing to  their  insiilatiny  power. 

1.  Gum  lac,  gutta  percha. 

2.  Amber. 

3.  Resins,  sulphur,  wax,  jet. 

4.  Glass,  and  all  vitrifactions. 

5.  Mica. 

6.  Diamond,  transparent  gems. 

7.  Raw  silk,  bleached  silk,  dyed 

silk. 

8.  Wool,  hair,  feathers. 

9.  Dry  paper. 

10.  Parchment,  leather. 

11.  Atmospheric  air,  when  dry. 

12.  All  dry  gases. 

13.  Baked  wood. 

14.  Porcelain  and  dry  marble. 

15.  Camphor,  Indian  rubber. 

16.  Lycopodium. 

17.  Dry  chalk,  lime,  phosphorus, 

18.  Ice  below  13°  Fahrenheit. 

19.  Many  dry,  transparent  crystals. 

20.  Oils,  dry  oxides  of  metals. 


214 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Conducting  substances  were,  at  one  time,  called  non-elec- 
trics, and  non-conductors  were  called  electrics ;  but  the  dis- 
tinction is  not  founded  on  fact,  because  conducting  substances, 
when  insulated,  will  yield  electricity  by  friction ;  and  beside*, 
the  capacity  of  a  substance  for  yielding  electricity  by  friction 
does  not  seem  to  depend  upon  the  insulating  or  non-conduct- 
ing power  of  the  substance. 

The  atmosphere  manifestly  belongs  to  the  class  of  non- 
conductors ;  if  this  had  not  been  the  case,  no  electrified  body 
could  have  retained  its  electricity  for  any  length  of  time. 
When  air  becomes  rarefied,  it  loses  its  insulating  property  ; 
thus,  an  electrified  body  soon  loses  its  electricity  when  placed 
in  the  exhausted  receiver  of  an  air  pump.  The  electric  fluid 
spreads  itself  in  a  thin  coating  over  the  surface  of  the  electri- 
fied body,  and  it  is  prevented  from  escaping  by  the  pressure  or 
tension  of  the  surrounding  air  ;  when  this  pressure  is  reduced 
beyond  a  certain  degree,  the  electricity  escapes  from  the  sur- 
face. 

ELECTROSCOPES. 

4.  Electroscopes  are  instruments  used  for  detecting  the 
presence  of  electricity  in  bodies ;  such  as  the  suspended  pith 
balls  represented  in  Fig.  3. 

By  means  of  an  electroscope  we  can 
readily  show  that  there  are  two  kinds 
of  electricity ;  the  one  being  called  pos- 
itive electricity,  and  the  other  negative 
electricity.  There  are  various  electro- 
scopes, but  the  following  one  is  easily 
made,  and  is  quite  delicate  enough  for 
all  ordinary  electrical  experiments. 

To  make  a  simjjle  electroscope.  — 
Take  a  narrow  strip  of  tin  foil ;  melt  T!i 
the  end  of  a  stick  of  sealing  wax ;  at- 
tach it  to  one  end  of  the  tin  foil,  and 
draw  the  wax  out  into  a  fine  thread,  as 
shown  in  Fig.  13,  where  T  represents 


Fig.  13. 


the  tin  foil,  and  W  the  sealing  wax ;  place  the  stick  of  sealing  wax  on 
the  mantel  shelf  M,  and  you  will  have  constructed  a  very  useful  elec- 


ELECTRICITY.  215 

troscope,  with  which  the  following  demonstrative  experiments  may  be 

successfully  performed. 

5.    There  are  two  kinds  of  electricity. 

Make  two  simple  electroscopes  ;  excite  a  stick  of  sealing 
wax,  and  also  a  dry  glass  tube ;  the  electricity  of  the  sealing 
wax,  which  is  said  to  be  negative,  will  be  different  from  the 
electricity  of  the  glass,  which  is  said  to  be  positive,  as  may  be 
shown  by  the  following  experiments  :  — 

Exp.  1.  Touch  the  strips  of  tin  foil  with  the  excited  glass  tube,  bring 
them  near  to  each  other,  and  they  will  fly  from  each  other,  as  shown  in 
No.  1,  Fig.  14.  Here  the  bodies  repel  each  other,  because  they  are  elec- 


No.2.  No.  3. 


trifled  in  the  same  way ;  that  is  to  say,  they  are  loth  in  a  state  of  positive 
or  phis,  -f-,  electricity. 

Exp.  2.  Perform  the  same  experiment  with  the  excited  sealing  wax, 
and  the  strips  of  tin  foil  will  repel  each  other,  as  shown  in  No.  3,  Fig. 
14.  Here  the  bodies  are  both  in  a  state  of  negative  or  minus,  — ,  elec- 
tricity. 

Exp.  3.  Touch  one  of  the  strips  of  tin  foil  with  the  excited  glass 
tube,  and  touch  the  other  strip  with  the  excited  stick  of  sealing  wax  ; 
bring  the  strips  thus  electrified  near  each  other  ;  they  will  be  powerfully 
attracted  to  each  other,  as  shown  in  No.  2,  Fig.  14,  thereby  proving 
that  the  electricity  generated  by  the  friction  of  glass  is  different  from 
the  electricity  generated  by  the  friction  of  sealing  wax. 

These  experiments  may  be  readily  performed  with  one  electroscope  in 
the  following  manner :  — 

Exp.  4.  Bring  the  excited  stick  of  sealing  wax  near  the  strip  of  tin 
foil ;  it  will  be  first  attracted,  and  then  it  will  remain  permanently  re- 
pelled. Any  other  excited  stick  of  sealing  wax,  or  any  excited  resinous 
substance,  will  repel  the  electrified  strip.  Now  bring  an  excited  glass 
tube  near  to  the  electrified  tin  foil ;  it  will  be  instantly  attracted. 

From  these  experiments  we  derive  the  following  law,  relative  to  the 
IAVO  kinds  of  electricity  :  — 


216         NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

Bodies  electrified  in  the  same  way  repel  one  another; 
bodies  electrified  in  different  ways  attract  one  another.  Or 
we  may  express  this  law  by  simply  stating,  that  like  electrici- 
ties repel,  and  unlike  attract. 

By  this  law  of  electrical  polarity  we  may  easily  ascertain 
to  which  kind  of  electricity  any  excited  body  belongs. 

Exp.  1.  Suppose  we  wish  to  know  whether  the  excited  brown  paper 
of  Exp.  5,  p.  208,  is  positive  or  negative. 

Touch  the  strip  of  tin  foil  T,  (Fig.  13,)  with  an  excited  stick  of  sealing 
wax,  and  the  tin  foil  will  be  negatively  electrified  :  now  bring  the  ex- 
cited brown  paper  near  to  the  strip  of  tin  ibil ;  it  is  repelled ;  therefore 
the  paper  is  electrified  in  the  same  manner  as  the  sealing  wax ;  that  is,  it 
is  in  a  state  of  negative  electricity. 

Or  we  may  proceed  as  follows :  Touch  the  tin  foil  of  the  electroscope 
with  an  excited  glass  tube ;  bring  the  excited  paper  near  to  the  tin  foil ; 
it  is  instantly  attracted,  thereby  showing  that  the  electricity  of  the  ex- 
cited paper  is  unlike  the  electricity  of  the  excited  glass ;  that  is,  the  elec- 
tricity of  the  paper  is  negative. 

Exp.  2.  Perform  the  same  experiment  with  excited  sulphur.  It  will 
be  found  to  possess  negative  electricity. 

Exp.  3.  Rub  a  glass  tube  with  a  black  cat's  skin  ;  test  the  electricity 
of  the  glass  ;  it  will  be  found  to  be  negative,  thereby  showing  that  the 
same  substance  may  be  positively  or  negatively  electrified,  according  to 
the  nature  of  the  rubber. 

Exp.  4.  Test  the  electricities  of  the  two  sheets  of  brown  paper  of 
Exp.  6,  p.  208  ;  the  upper  sheet  will  be  negative,  while  the  bottom  sheet 
will  be  positive. 

After  separating  the  sheets  of  brown  paper,  place  them  on  opposite 
sides  of  the  tin  foil ;  it  will  fly  with  great  rapidity  backwards  and  for- 
wards from  the  one  sheet  of  paper  to  the  other. 

Exp.  5.  Hold  an  excited  stick  of  sealing  wax  and  an  excited  glass 
rod  near  to  the  tin  foil  of  the  electroscope ;  the  tin  foil  will  fly  backwards 
and  forwards  from  the  one  to  the  other.  Perform  the  same  experiment 
with  the  flying  feather  of  Exp.  3,  p.  207. 

6.  Electricity  remains  on  the  surface  of  a  non-conductor 
when  it  is  electrified ;  that  is  to  say,  the  electrified  fluid  does 
not  pass  from  one  part  of  the  surface  to  another  part. 

Experiment.  Excite  the  whole  surface  of  a  piece  of  sealing  wax  with 
dry  silk ;  run  the  fore  finger  down  one  side  of  the  excited  sealing  wax; 
touch  the  tin  foil  of  the  electroscope  with  that  side  of  the  sealing  wax 


ELECTRICITY.  ,    217 

from  which  the  electricity  has  not  been  taken  away,  then  the  tin  foil  will 
be  repelled  ;  turn  the  sealing  wax  round,  then  the  tin  foil  will  no  longer 
be  repelled.  Here  it  will  be  seen  that  the  electricity  does  not  spread 
itself  from  one  side  of  the  sealing  wax  to  the  other  side. 

7.  The  electricity  of  the  rubber  is  different  from  the  elec- 
tricity of  the  body  which  is  rubbed. 

• 

Exp.  1.  Lay  a  piece  of  dry  silk  upon  the  table,  and  rub  it  with  a  stick 
of  sealing  wax ;  lift  up  the  excited  silk  by  one  corner,  and  touch  the 
tin  foil  of  the  electroscope  with  it,  then  the  tin  foil  will  be  charged  with 
positive  electricity ;  bring  the  excited  sealing  wax  near  to  the  tin  foil, 
and  it  will  be  powerfully  attracted,  thereby  showing  that  while  the  seal- 
ing wax  is  in  a  negative  state  of  electricity,  the  silk  is  in  a  positive  state. 

Exp.  2.  Tie  a  piece  of  silk  or  flannel  to  the  end  of  a  stick  of  sealing 
wax ;  rub  a  warm  plate  of  glass  with  the  insulated  silk,  taking  care  to 
hold  the  rubber  by  the  insulating  handle ;  test  the  electricity  of  the  rub- 
ber and  the  excited  glass  by  means  of  the  electroscope,  and  it  will  be 
found  that  the  silk  rubber  is  negative,  while  the  glass  is  positive. 

Exp.  3.  Rub  a  sheet  of  brown  paper  in  the  same  manner.  In  this 
case  the  silk  rubber  will  be  positive,  and  the  sheet  of  paper  negative. 

THEORIES    OP    ELECTRICITY. 

8.  These  experiments  led  some  philosophers  to  consider 
that  there  was  only  one   electric  fluid,  and   that  it  existed  in 
the  glass,  which   was  rubbed,  in  excess,  or  in  a  plus  state, 
while  it  existed  in   the  rubber  in  deficiency,  or  in  a  minus 
state.  .  According  to  this  theory,  the   friction    deprives  the 
rubber  of  a  portion  of  its  natural  electricity,  and  transmits  it 
to  the  glass,  which  thereby  receives  more  than  its  natural 
share.     This  explains  the  use  of  the  terms  positive  and  nega- 
tive  electricity.     However,  as  we  shall  afterwards  show,  it 
seems  to  be  more  simple  for  us  to  regard  electricity  as  con- 
sisting of  two  fluids,  which  mutually  attract  each  other,  but, 
at  the  same  time,  each  fluid  is  self-repellent  —  that  is  to  say, 
its  own  particles  repel  one  another.     The  kind  of  fluid  ex- 
cited from  glass  and  analogous  bodies  is  called  vitreous  ;  and 
that  from  sealing  wax  and  the  like,  resinous  electricity.     The 
vitreous  answers  to  the  positive,  and  the  resinous  to  the  nega- 
tive.   This  theory  fully  accounts  for  the  electrical  attractions 

19 


218          NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

and  repulsions ;  for  when  the  electric  fluids  in  two  "bodies  are 
unlike,  the  bodies  attract  each  other,  by  virtue  of  the  mutual 
attraction  of  the  two  fluids ;  and,  on  the  contrary,  when  the 
electric  fluids  in  the  two  bodies  are  like,  the  bodies  repel  each 
other,  by  virtue  of  the  repellent  property  of  the  particles  of 
the  same  fluid.  When  equal  portions  of  the  two  fluids  unite, 
they  neutralize  each  other,  and  the  electricity  is  then  in  a  neu- 
tral or  quiescent  state,  which  is  the  usual  state  in  which  elec- 
tricity exists  in  bodies.  Friction  disturbs  the  equilibrium  of 
the  two  fluids,  by  separating  the  one  from  the  other:  the  pos- 
itive fluid  attaches  itself  to  the  glass,  while  the  negative  fluid 
attaches  itself  to  the  rubber.  The  two  fluids,  in  the  natural 
state  of  bodies,  as  it  were  hold  each  other  in  a  state  of  inac- 
tion, and  electricity  is  then  said  to  be  latent  or  hidden. 

THE  SINGLE  FLUID  THEORY  was  adopted  by  Franklin, 
and  after  him  by  most  of  the  English  electricians,  until  very 
recently,  when  THE  THEORY  OF  Two  FLUIDS,  as  above  ex- 
plained, which  had  been  generally  adopted  on  the  continent, 
became  more  popular  amongst  us.  It  must,  however,  be  re- 
membered that  the  great  use  of  theory  in  this  subject,  is  to 
group  and  classify  the  vast  accumulation  of  facts  which  have 
been  brought  to  light. 

CONDUCTION   AND    INDUCTION. 

9.  When  the  electric  fluid  is  transmitted  from  one  body  to 
another  through  the  medium  of  an  insulated  conductor,  it  is 
said  to  be  conveyed  by  conduction  ;  but  when  electricity  is 
transmitted  from  one  body  to  another  at  some  distance  from 
it  without  receiving  a  spark,  it  is  said  to  be  by  induction. 

Experiment.  Support  a  tea  spoon  B  C,  or  any  thick  metal  wire,  upon 
a  stick  of  sealing  wax  S :  this  can  easily  be  done  by  melting  the  wax, 
and  fixing  the  spoon  to  it,  as  shown  in  Fig.  13,  page  214.  The  spoon 
will  thus  form  an  insulated  conductor. 

(1.)  Hold  the  conductor  B  C  by  the  insulating  stick  S;  bring  the 
extremity  C  near  to  the  tin  foil  T  of  the  electroscope ;  then  touch  the 
opposite  extremity  B  with  an  excited  stick  of  sealing  wax ;  the  tin  foil 
will  be  attracted  and  then  repelled.  Here  the  metal  B  C  conducts  or 


ELECTRICITY.  219 

conveys  the  electricity  from  the  sealing  wax  A  to  the  tin  foil  T.  This 
is  an  example  of  conduction;  B  C  may  be  any  conducting  substance. 

If  the  intervening  substance  B  C  were  glass,  or  any  other  non-con- 
ductor, the  tin  foil  would  not  be  affected  by  the  contact  of  A  with  the 
extremity  B. 

(2.)  Bring  the  extremity  C  of  the  conductor  at  the  distance  of  about 
half  an  inch  from  the  tin  foil ;  hold  the  excited  stick  of  sealing  wax  A 
at  about  the  same  distance  from  the  extremity  B  ;  then  T  will  be  elec- 
trified negatively,  which  can  readily  be  tested  in  the  usual  way.  This 
is  an  example  of  electrical  induction.  Take  A  away,  and  all  signs  of 
electricity  will  have  disappeared  from  the  conductor  B  C.  Here  the 
electricity  is  conveyed  or  transmitted  from  the  electrified  body  to  the  tin 
foil  through  the  air,  and  not  by  the  contact  of  the  conductor  with  the 
electrified  body.  Electrical  induction,  then,  takes  place,  when  electricity 
is  transmitted  from  one  body  to  another  body  at  some  distance  from  it. 
The  phenomena  here  exhibited  may  be  explained  as  follows  :  — 

The  negative  electricity  on  A  repels  the  negative  electricity  in  the 
conductor  B  C,  and  attracts  its  positive  electricity  ;  the  consequence  is, 
the  equilibrium  of  the  two  fluids  in  the  conductor  is  destroyed,  the  neg- 
ative fluid  flies  towards  the  extremity  C,  and  the  positive  fluid  is  at- 
tracted towards  the  extremity  B.  Hence  the  tin  foil  is  first  attracted 
towards  C,  and  then  repelled  from  it.  And,  upon  withdrawing  the 
conductor,  the  tin  foil  will  remain  electrified  negatively.  To  prove 
this,  bring  an  excited  stick  of  sealing  wax  towards  the  tin  foil  T,  and  it 
will  be  repelled. 

(3.)  Perform  the  same  experiment  with  an  excited  glass  tube  A.  In. 
this  case  the  extremity  C  will  be  positive,  and  the  tin  foil  will  be 
charged  with  positive  electricity. 

(4.)  Repeat  Exp.  2,  and  before  taking  the  electrified  sealing  wax  A 
away,  first  touch  C,  and  then  take  A  away ;  the  conductor  will  remain 
positively  electrified,  which  will  be  shown  by  its  now  attracting  T.  If 
we  touch  the  extremity  B,  the  conductor  will  remain  electrified  nega- 
tively. 

These  effects  may  be  readily  explained.  When  we  touch  the  ex- 
tremity C,  we  take  away  the  free  negative  electricity,  and  then  when  A 
is  taken  away  an  excess  of  positive  electricity  remains  in  the  conductor. 
In  like  manner  when  we  touch  the  extremity  B  we  take  away  the  free 
positive  electricity,  and  then  when  A  is  taken  away  the  conductor  B  C 
remains  charged  with  negative  electricity.  The  truth  of  these  results 
may  be  readily  verified  in  the  usual  way.  Observe  that  the  tin  foil  T 
will  always  remain  charged  with  the  electricity  of  the  extremity  C  of 
the  conductor. 

Electrical  attractions  are  readily  explained  upon  the  principle  of 


^..,A,,.li,  „  .  s? 


220          NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

induction :  by  the  action  of  induction,  the  body  which  is  attracted  is  in  a 
different  state  of  electricity  from  that  of  the  body  charged  with  the 
electricity. 

Tattfs  simple   Gutta  Percha  Electrophorus. 

10.  The  following  apparatus  depends  upon  the  principle  of  induc- 
tion :  — 

Take  a  toy  tin  plate,  costing  one  penny;  heat 
the  bottom  of  the  plate  over  the  flame  of  a  candle, 
and  fix  a  stick  of  sealing  wax  A,  as  shown  in  Fig. 
15,  to  its  upper  surface ;  lay  a  sheet  of  gutta 
pcrcha  (or  a  sheet  of  warm  brown  paper,  as  the  case  /™ 
may  be)  upon  a  smooth  table,  and  excite  the  sheet  j  ^ 

in  the  usual  way ;  place  the  tin  plate  upon  the  sur-  Fig.  15. 

face  of  the  gutta  perch  a,  and,  after  touching  the 
plate  with  the  finger,  lift  it  off  the  gutta  percha  by  means  of  the  insu- 
lating handle  ;  apply  the  knuckle  to  the  tin  plate,  and  a  spark  of  posi- 
tive electricity  will  be  received.    This  may  be  repeated  for  about  a  hun- 
dred times,  without  any  sensible  diminution  in  the  size  of  the  spark. 

Here  the  friction  of  the  gutta  percha  generates  negative  electricity ; 
and  therefore,  when  we  touch  the  plate,  we  take  away  a  certain  portion 
of  negative  electricity  from  it,  and  consequently,  when  the  plate  is  raised, 
it  must  contain  an  excess  of  positive  electricity. 

In  order  to  give  a  continuous  charge  to  a  conductor,  place  the  insu- 
lated tea  tray,  represented  in  Fig.  9,  directly  above  the  edge  of  the 
plate  A  of  the  electrophorus,  so  that  when  the  plate  is  lifted  off  the 
sheet  of  gutta  percha,  it  shall  strike  against  the  edge  of  the  tea  tray. 
In  this  way  a  rapid  succession  of  sparks  will  be  transmitted  to  the  tea 
tray,  which  will  consequently  become  powerfully  charged  with  positive 
electricity.  An  electrical  jar,  having  its  knob  placed  near  to  the  edge 
of  the  tea  tray,  will  be  soon  charged  with  positive  electricity. 

11.  By  means  of  this  electrophorus,  the  following  demonstrative  ex- 
periments may  be  readily  performed  :  — 

To  show  that  pointed  conductors  draw  off  electricity  from  an  electrified 


Fig.  16, 


ELECTRICITY.  221 

body  :  Place  a  common  toasting  fork  upon  a  dry  wine  glass,  as  shown 
in  Fig.  16  ;  bring  the  electrified  plate  of  the  electrophorus  near  to  the 
points  of  the  fork,  then  a  spark  may  be  taken  from  its  opposite  extremity. 
To  exhibit  electrical  induction  and  conduction :  Place  a  poker  upon  a 
dry  wine  glass,  as  shown  in  Fig.  17  ;  touch  one  extremity  of  the  poker 
with  the  electrified  plate  of  the  electrophorus,  and  a  spark  may  be  re- 
ceived from  the  opposite  extremity,  thereby  showing  that  the  iron  is  a 
conductor  of  electricity. 


Fig.  17. 

To  show  the  induction  of  electricity  in  this  case,  bring  the  electrified 
plate  of  the  electrophorus  near  to  one  extremity  D  of  the  poker,  but  not 
so  near  as  to  transmit  a  spark  ;  then  a  spark  of  positive  electricity  may 
be  received  from  the  opposite  extremity  C. 

The  tin-foil  needle  electroscope*  —  In  order  to  render  the  law  of  induc- 
tion more  apparent,  construct  an  electroscope  A  P  J.  (Se"e  Fig.  17.) 
Take  a  strip  of  card  paper  A  P  B  about  six  inches  long,  and  half  an 
inch  wide  ;  attach  narrow  strips  of  tin  foil  to  the  extremities  of  the  card 
paper,  by  means  of  insulating  knobs  of  sealing  wax,  and  balance  the 
card  paper,  on  a  small  indentation  made  at  its  centre,  on  the  point  P  of 
a  pin  passed  through  a  cork  and  placed  on  the  top  of  a  wine  glass  J. 
With  the  view  of  adjusting  the  balance,  two  small  rings  of  Indian  rub- 
ber are  placed  on  the  card,  one  on  each  side.  This  will  form  a  delicate 
electroscope,  which  may  be  used  in  conducting  some  interesting  experi- 
ments hereafter  to  be  described. 

Bring  the  strip  of  tin  foil,  of  this  electroscope,  near  to  the  one  extrem- 
ity of  the  poker,  (see  Fig.  17,)  and  then  bring  the  insulated  plate  of 
the  electroscope  near  to  the  other  extremity,  and  the  needle  will  be  de- 
ected,  the  tin  foil  being  electrified  with  positrse  electricity:  touch  the 
extremity  C  with  the  finger,  then  take  away  the  plate  of  the  elcctropho- 
rus,  and  the  needle  of  the  electroscope  will  return  to  its  first  position ; 
for  the  poker  will  be  left  in  a  negative  state  of  electricity,  while  the  tin 
foil  of  the  electroscope  will  be  in  a  positive  state ;  and  so  on  to  other 
experiments  of  this  kind,  illustrating  the  great  law  of  electrical  induction. 
19* 


222 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


A  remarkable  case  of  induction.  —  Place  a  small  tea  tray  T  upon  a 
dry  wine  glass  J,  and  upon  this  tray  place  the  electroscope  just  described, 
as  shown  in  Fig.  18  ;  charge  the  tea  tray  T  with  positive  electricity,  by 


Fig.  18. 

means  of  the  electrophorus,  described  at  page  220 ;  then,  because  the 
tin  foil  C  B  is  insulated  by  the  action  of  induction,  the  lower  extremity 
C  will  be  negative,  while  the  upper  extremity  B  is  positive.  Touch  the 
upper  extremity  B  of  the  tin  foil  C  B,  and  it  will  remain  charged  with 
negative  electricity ;  now  bring  the  hand  over  the  tray,  near  to  the  ex- 
tremity C  of  the  tin  foil,  and  it  will  be  instantly  repelled,  giving  the 
appearance  of  the  hand  as  being  negatively  electrified,  which,  in  fact,  it 
really  is  from  the  induction  of  the  tray  T. 

To  electrify  a  tin  plate,  either  negatively  or  positively,  by  means  of  the 
electrophorus.  —  (1.)  Place  the  tin 
plate  A  upon  a  dry  wine  glass,  (see 
Pig.  19  ;)  charge  the  plate  B  of  the 
electrophorus  positively,  after  the 
manner  described  at  page  220,  and 
bring  it  near  to  the  insulated  plate 

A,  (without  allowing  a  spark   to 
pass  from  the  one  to  the  other;) 
touch  the  plate  A,  and  a  spark  of 
positive  electricity  will  be  received 
from  the  inductive   action   of  the 
plate  B  ;  first  take  away  the  knuc- 
kle, and  then  take  away  the  plate 

B,  and  the  plate  A  will   remain  Fig.  19. 
charged  with  negative  electricity. 

(2.)  To  electrify  the  insulated  plate  A  positively,  simply  touch  it  with 
the  charged  plate  B  of  the  electrophorus. 


I 


ELECT1UCITY. 


To  exhibit  the  dancing  balls.  —  Put  a  few  small  pith  balls  into  the 
plate  A,  (see  Fig.  19  ;)  bring  the  electrified  plate  B  over  them,  as  shown 
in  the  figure,  and  they  will  appear  to  jump  up  and  down. 

The  electric  bell.  —  Place  a  damp  wine 
glass  C  (as  shown  in  Fig.  20)  near  to  the 
insulated  plate  A ;  suspend  a  small  brass 
ball  or  button  D  from  a  dry  silk  thread 
between  the  glass  and  the  plate;  elec- 
trify time  after  time  the  plate  A  by 
means  of  the  electrophorus ;  and  the  ball 
D  will  oscillate  between  the  plate  and 
the  glass,  thereby  producing  a  tinkling 
sound. 


Fig.  20. 


The  electrical  pendulum.  —  This  instrument  is  represented  in  Fig.  21. 
A  and  B  are  two  insulated  plates  ;  the  one  is  charged  with  positive,  and 
the  other  with  negative  electricity  ;  E  F  is  a  strip  of  card  paper,  having 


Fig.  21. 

a  pin  P  passed  through  it,  and  a  piece  of  pith  E  attached  to  its  upper 
extremity  by  means  of  an  insulating  knob  of  sealing  wax  ;  the  pin  P  of 
the  pendulum  E  F  is  supported  on  the  edges  of  two  wine  glasses,  which 
are  not  shown  in  the  cut  The  apparatus  is  adjusted  so  as  to  allow  the 
insulated  pith  E  to  oscillate  between  the  edges  of  the  plates  A  and  B. 
With  the  view  of  causing  the  lower  extremity  F  to  preponderate,  a  small 
sliding  ring  of  Indian  rubber  is  placed  on  the  portion  P  E  of  the  pen- 
dulum. 

The  electrical  hammer.  —  This 

simple  piece  of  apparatus  is  rep-      , . ^ 

resented  in  Fig.   22.     Here  the  ~~S^ 

pendulum  E  F  of  the  apparatus 
just  described  is  supported  in  a 
horizontal  position  in  the  man- 
ner already  described;  the  pith  Fi9"  22< 
knob  E,  in  this  case,  oscillates 
between  the  electrified  plate  A  and  a  conductor  D. 


224  NATURAL    AND    EXI'ERIMKNTAL    nilLOSOPHY. 

Tote's  electrical  revolver.  —  This  simple  and  interesting  piece  of  appa- 
ratus is  represented  in  Fig.  23.  F  and  E  are  two  insulated  plates  charged 
with  different  kinds  of  electricity,  (see  p.  222  ;)  A  B  J  is  the  tin  foil 
electroscope,  described  at  page  221,  placed  between  the  electrified  plates 


Fig.  23. 

E  and  F,  so  that  the  lower  extremities  of  the  strips  of  tin  foil  may  nearly 
touch  the  plates  E  and  F.  When  the  plates  are  electrified,  the  electrical 
needle  A  B  rapidly  revolves  upon  its  centre  P  ;  the  plates  charge  the 
insulated  strips  of  tin  foil  as  they  pass  them,  so  that  the  plates  attract 
the  strips  of  tin  foil  when  they  are  on  one  side,  and  repel  them  when 
they  are  on  the  other  side.  The  charge  of  the  plates  must  be  from  time  to 
time  renewed.  The  action  of  the  instrument  is  improved  by  placing  a 
conducting  knob  Q,  midway  between  the  two  plates  E  and  F,  so  as  to 
discharge  the  electricity  of  the  strips,  as  they  pass  the  conducting  knob. 
All  the  apparatus  we  have  hitherto  described  may  be  easily  construct- 
ed, at  a  very  small  cost,  by  any  person  of  ordinary  skill  and  patience. 

ELECTRICAL    MACHINES. 

12.  Electrical  machines  are  used  for  generating  electricity 
by  friction  on  a  large  scale.  They  consist  of  three  leading 
parts.  The  rubber  is  a  soft  hair  cushion,  covered  with  leather 
or  with  some  substance  which  readily  generates  electricity  by 
friction.  The  body  on  which  the  rubber  acts  is  either  a  glass 
cylinder  or  a  circular  glass  plate,  which  turns  upon  an  axis. 
The  receiver  of  the  electricity  is  called  the  prime  conductor  ; 
it  is  a  thin  brass  cylinder,  or  a  brass  rod,  mounted  on  a  glass 
pillar,  or  some  insulating  material.  The  action  of  an  electri- 
cal machine  is  simply  this  :  the  glass  cylinder,  or  the  glass 
plate,  as  the  case  may  be,  upon  being  turned,  rubs  against  the 
cushion,  and  thereby  generates  electricity  upon  the  surface 


ELECTRICITY.  225 

of  the  glass,  which  is  continually  carried  round  to  the  prime 
conductor. 

The  common  Cylindrical  Machine. 

13.  Fig.  24  represents  an  electri- 
cal machine  of  this  kind.  The  glass 
cylinder  A  B,  which  rests  on  an 
axis  passing  through  C,  is  made  to 
revolve  by  means  of  the  wheels  C 
and  D  connected  by  a  band,  the 
wheel  D  being  turned  by  means  of 
the  handle  R ;  the  cushion  H,  which 
rubs  against  the  cylinder,  is  mount- 
ed on  a  glass  pillar  I,  which  slides 
in  a  groove  at  the  foot,  for  the  pur  ^* 

pose  of  adjusting  the  pressure  upon  the  cylinder ;  the  chain  K  L  con- 
nects the  cushion  with  the  ground  ;  a  flap  B  of  varnished  silk  passes 
from  the  cushion  over  the  cylinder,  for  the  purpose  of  preventing  the 
escape  of  the  electricity  into  the  air ;  the  prime  conductor  M  N,  mount- 
ed on  the  glass  pillar  O  P,  has  a  row  of  points  projecting  from  the  ex- 
tremity M,  and  coming  nearly  in  contact  with  the  surface  of  the  glass 
cylinder.  As  glass  is  liable  to  collect  moisture  on  its  surface,  it  is  usual 
to  cover  all  the  insulating  pillars,  as  well  as  all  those  parts  of  the  cylin- 
der which  do  not  touch  the  cushion,  with  a  coating  of  varnish,  which 
has  a  higher  insulating  property  than  glass. 

Fig.  25  shows  the  construction  of  the  cushion  ;  where  H  H 

is  the  rubber,  with  an  adjusting  spring  fixed  behind  it,  for 
keeping  it  continually  pressed  against  the  cylinder  ;  K  the 
brass  knob,  or  ball,  for  attaching  the  chain. 

Fig.  26  shows  the  form  of  the  row  of  points  attached  to 
the  prime  conductor. 

When  the  cylinder  is  turned  round  by  the  handle  R,  pos- 
itive electricity  is  generated  on  the  surface  of  the  cylinder, 
and  negative  electricity  on  the  cushion.  The  latter  is  car- 
ried off  by  the  chain  to  the  ground.  The  positive  electri- 
city  is  carried  round  to  the  points  of  the  prime  conductor, 
where  it  acts  by  induction  on  the  natural  electricity  in  the 
conductor  —  that  is,  by  attracting  the  negative  fluid,  and 
repelling  the  positive.  The  negative  fluid,  escaping  by  the 
points,  unites  writh  the  positive  fluid  on  the  cylinder,  and 
thereby  restores  the*  surface  of  the  cylinder  to  its  natural 
state,  so  that  when  it  arrives  again  at  the  rubber  it  is  pre- 
pared for  another  charge  of  positive  fluid  ;  at  the  same  time  Fig.  26. 


226 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


the  prime  conductor  is  left  charged  with  positive  electricity.  Accord- 
ing to  this  theory,  the  negative  electricity  of  the  conductor  is  con- 
tinually passing  off  by  the  chain  attached  to  the  cushion,  which  con- 
stantly keeps  the  conductor  charged  with  positive  electricity.  By 
detaching  the  chain  from  the  cushion,  and  placing  it  on  the  prime  con- 
ductor, we  are  able  to  charge  the  cushion  with  negative  electricity. 

With  the  view  of  increasing  the  efficiency  of  the  machine,  the  cush- 
ion is  covered  with  an  amalgam  of  zinc  and  tin.  According  to  Singer, 
the  best  composition  of  the  amalgam  is  two  parts  by  weight  of  zinc,  one 
of  tin,  and  six  of  mercury.  The  mercury  is  added  to  the  mixture  of 
the  zinc  and  tin  when  in  a  fluid  state,  and  the  whole  is  then  shaken  in 
a  wooden  box  until  it  is  cold ;  it  is  then  reduced  to  a  powder,  and  mixed 
with  a  sufficient  quantity  of  lard  to  reduce  it  to  the  consistency  of  paste. 
A  thin  coating  of  this  paste  is  spread  over  the  cushion ;  but  before  this 
is  done,  all  the  parts  of  the  machine  should  be  carefully  cleaned  and 
warmed.  Black  spots  and  lines  are  readily  taken  from  the  glass  by  ap- 
plying a  rag  dipped  in  spirits  of  wine ;  and  the  efficiency  of  the  machine 
is  greatly  promoted  by  applying  with  the  hand  a  piece  of  leather  cov- 
ered with  amalgam  to  the  cylinder. 

The  common  Plate  Machine. 

14.  Fig.  27  represents  a  machine  of  this 
kind.  A  B  is  a  circular  plate  of  glass, 
turning  on  a  horizontal  axis  C  by  means  of 
the  winch  or  handle  D  ;  the  plate  is  em- 
braced at  E  by  two  cushions,  the  pressure 
of  which  is  adjusted  by  screws  ;  two  simi- 
lar' cushions  are  placed  at  E' ;  flaps,  pro- 
ceeding from  the  cushions,  cover  the  glass 
at  the  spaces  shown  in  the  figure  to  about 
half  an  inch  from  the  points  on  each  side 
of  the  conductor  ;  the  conductor  P  O  M  F 
is  a  small  brass  tube,  or  cylinder,  bent  so  as 
to  suit  the  plate,  and  supported  by  a  glass  rod  F'  M  attached  to  the 
upright  frame  E  ;  P  Q,  running  parallel  to  the  surface  of  the  plate,  is 
that  part  of  the  conductor  which  carries  the  points,  and  a  similar  bent 
branch  with  points  is  formed  at  F.  When  the  handle  D  is  turned  in 
the  direction  of  the  arrow,  the  cushions  at  the  top,  as  well  as  those  at  the 
bottom,  generate  electricity  ;  the  points  at  F  receive  the  electricity  gen- 
erated by  the  cushion  E,  whilst  those  at  P  O  receive  the  electricity  gen- 
erated by  E'.  In  order  to  prevent  the  escape  of  electricity,  all  the 
extremities  of  the  conductor  are  terminated  in  brass  balls  or  globes.  The 


Fig.  27. 


ELECTRICITY. 


227 


principle  on  which  this  machine  acts  is  precisely  the  same  as  that  of  the 
common  cylindrical  machine.  This  machine  is  more  powerful  than  the 
cylindrical  one ;  but  the  difficulty  of  insulating  the  rubbers,  so  as  to 
obtain  the  negative  fluid,  is  certainly  an  objection  to  it. 

The  Haerlem  plate  machine,  repre- 
sented in  Fig.  28,  fully  remedies  this 
deficiency  in  the  common  plate  ma- 
chine. The  glass  plate  is  fixed  to  the 
axis  D ;  the  two  cushions  are  insulated 
on  glass  pillars  E  and  F ;  C  B  C'  is  the 
bent  arm  of  the  prime  conductor,  armed 
with  points,  and  insulated  on  tjie  glass 
pillar  G  ;  in  order  to  connect  the  cush- 
ions with  the  ground,  there  is  a  bent 
or  semicircular  conductor,  similar  to 
C  B  C',  proceeding  from  the  axis  at 
D,  and  reaching  to  the  balls  of  the  two 
cushions. 

"When  it  is  required  to  charge  the 
conductor  B  with  negative  electricity, 

the  semicircular  rod  C  B  C'  is  moved  into  a  horizontal  position,  thereby 
bringing  the  points  opposite  to  the  two  cushions ;  at  the  same  time  the 
other  semicircular  rod,  on  the  opposite  side  of  the  plate,  is  moved  round 
into  a  vertical  position,  thereby  bringing  its  points  at  the  top  and  bottom 
parts  of  the  plate. 

Fig.  29  represents  another  form  of  the  plate  machine ;  where  C  is  the 


Fig.  28. 


Fig.  29. 


228 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


prime  conductor,  mounted  on  the  glass  pillar  K  ;  Gr  G  the  glass  plate ; 
g  the  winch ;  B  B,  the  cushions ;  S  S'  the  flaps,  &c. ;  E  a  quadrant  elec- 
trometer inserted  in  the  conductor,  to  determine  the  quantity  of  elec- 
tricity with  which  it  may  be  discharged ;  and  a  a,  b  b  an  apparatus  sus- 
pended from  the  conductor  to  illustrate  the  principle  of  electrical  attrac- 
tion and  repulsion. 


APPENDAGES   TO    ELECTRICAL   MACHINES. 

15.  The  insulating  stool,  represented 
in  Fig.  30,  consists  of  a  board  of  hard, 
well-baked  wood,  supported  on  glass 
legs  covered  with  varnish.  It  is  useful 
for  insulating  any  body  charged  with 
electricity ;  for  instance,  a  person  may 
stand  upon  the  stool  and  become  charged 
with  electricity,  upon  being  put  in  con- 
nection with  the  prime  conductor  of  the  electrical  machine. 

Discharging  rods  are  brass  rods  terminating  with  balls,  or  with  points 
fixed  to  glass  handles.  With  these  rods,  electricity  may  be  taken  from 
a  conductor  without  allowing  the  electrical  charge  to  pass  through  the 
body  of  the  operator. 

Fig.  31  represents  a  common  discharger  ;  where  A  is  the  glass  handle, 
C  E  D  the  brass  rod,  C  and  D  the  balls. 


Fig.  30. 


Fig.  31. 


Fig.  32. 


Fig.  32  represents  a  double-handled  jointed  discharger  ;  where  A  and 
B  are  the  glass  handles,  E  the  joint,  &c. 

The  Ley  den  jar  consists  of  a  glass  cylinder,  or  wide-mouthed  bottle,  T, 


ELECTRICITY. 


229 


(see  Fig.  31,)  both  surfaces  of  which  are  coated  with  tin  foil  up  to  about 
three  inches  of  the  top.  The  coating  of.  tin  foil  on  the  outside  of  the 
bottle  is  called  the  outer  coating  ;  the  other,  on  the  inside,  is  called  the 
inner  coating.  Electricity  is  transmitted  to  this  coating  by  means  of  a 
metal  rod  K  a,  terminated  at  the  upper  extremity  by  a  knob  K,  and  at 
the  lower  extremity  by  a  chain  which  comes  into  contact  with  the  inner 
coating  of  the  jar.  The  rod  is  fixed  by  passing  tightly  through  a  wooden 
plug,  which  fits  firmly  into  the  neck  of  the  jar.  Those  portions  of  the 
glass  which  are  not  coated  with  the  tin  foil  are  covered  over  with  a  thick 
coating  of  wax,  to  prevent  a  reunion  between  the  electricity  of  the  outer 
coating  and  that  of  the  inner  coating.  When  the  jar  is  to  be  charged,  it 
is  held  in  the  hand  by  the  outer  coating,  and  the  knob  K  is  brought  near 
to  the  conductor  of  the  electrical  machine.  While  spark  after  spark  of 
positive  electricity  enters  the  jar,  the  positive  electricity,  on  the  principle 
of  induction,  is  driven  off  from  the  outer  coating ;  so  that  while  the 
inner  coating  becomes  charged  with  positive  electricity,  the  outer  coating 
becomes  charged  with  negative  electricity  in  a  manner  which  will  be 
hereafter  more  fully  explained.  When  the  jar  is  to  be  discharged,  the 
operator,  holding  the  discharging  rod  by  the  glass  handle  A,  brings  one 
knob  C  in  contact  with  the  outer  coating,  and  then  gradually  brings  the 
other  knob  D  near  to  the  knob  K  of  the  jar ;  the  reunion  of  the  two 
fluids  (the  positive  from  the  inner  coating,  and  the  negative  from  the 
outer  coating)  takes  place  between  the  two  knobs  D  and  K  with  a  bright 
spark  and  a  snapping  noise. 

The  universal  discharge,   represented  in  Fig.  33,  consists  of  a  dry  deal, 


33. 


on  which  two  glass  pillars  A  and  B  are  fixed  ;  two  brass  rods  a  b  and 
a  b,  capable  of  turning,  on  a  ball  and  socket  joint,  in  any  direction,  and 
also  capable  of  sliding  in  the  top  balls  ;  the  knobs  a  a  are  applied  to  a 
wooden  table  t,  which  admits  of  being  raised  or  depressed  by  means  of 
an  adjusting  screw  v  ;  a  narrow  strip  of  ivory  is  inlaid  across  the  table ; 
the  knobs  a  a  may  be  screwed  off,  and  replaced  by  points  or  by  forceps. 
This  piece  of  apparatus  is  much  used  for  passing  strong  charges  of  elec- 
tricity through  any  substance. 
20 


230 


NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 


The  quadrant  electrometer.  —  This  instrument  is  used  for 
indicating  the  quantity  of  electricity  accumulated  in  the 
prime  conductor  of  the  machine.  It  consists  of  a  vertical 
stem  or  rod,  which  admits  of  being  inserted  in  a  hole  made 
in  the  prime  conductor  ;  to  the  side  of  this  stem  is  fixed  a 
graduated  quadrant,  carrying  a  light  needle  or  rod,  termi- 
nated by  a"  pith  ball ;  this  light  needle  turns  on  a  pivot  O 
fixed  in  the  centre  of  the  quadrant.  "When  the  machine  is 
not  in  action,  the  light  needle  hangs  parallel  to  the  vertical 
stem  ;  but  when  the  machine  is  worked,  the  needle  is  repelled  from  the 
stem,  and  the  height  to  wliich  it  ascends  indicates  the  amount  of  elec- 
tricity accumulated  in  the  prime  conductor. 


Fig.  34. 


16.  A  FEW  EASY  EXPERIMENTS  WITH  THE  ELECTRICAL 
MACHINE.- 

Exp.  1.   Work  the  machine ;  bring  your  knuckle  near  to  the  prime 
conductor ;  a  vivid  and  instantaneous  flash,  accompanied  with  a  snap- 


Fig.  35. 


ELECTRICITY.  231 

ping  noise,  passes  between  the  conductor  and  your  hand,  which  produces 
a  slightly  painful  sensation  :  this  is  the  electric  spark. 

A  spark  will  be  communicated  to  any  conductor.  Hold  a  stick  of 
sealing  wax,  or  any  other  non-conductor,  to  the  prime  conductor :  no 
spark  will  be  received. 

Exp.  2.  Fix  the  quadrant  electrometer  on  the  prime  conductor ;  work 
the  machine,  and  observe  to  what  height  the  pith  ball  is  repelled.  Hold 
the  point  of  a  sewing  needle  near  to  the  conductor :  the  pith  ball  of  the 
electroscope  instantly  falls.  Take  sparks  from  the  conductor :  the  pith 
ball  falls  at  the  instant  each  spark  is  taken. 

Exp.  3.  Let  a  boy  stand  on  the  insulating  stool,  and  let  him  place  one 
of  his  hands  on  the  prime  conductor ;  work  the  machine ;  take  sparks 
from  his  body :  see  how  he  winces  from  the  smarting  sensation  they 
produce,  especially  when  taken  through  his  clothes  !  (See  Fig.  35.) 

Exp.  4.  Charge  a  Ley  den  jar  fully,  and  discharge  it  with  the  jointed 
discharging  rod  :  see  what  a  vivid  spark  it  gives  ! 

Charge  the  Leydeii  jar  (with  about  half  a  dozen  sparks ;)  grasp  the 
outer  coating  with  one  hand,  and  touch  the  knob  with  the  other.  The 
electric  fluid,  in  passing  through  your  body,  gives  you  what  is  called  an 
electric  shock. 

Let  a  few  boys  form  a  ring  by  taking  hold  of  each  other's  hands ;  let 
the  first  boy  in  the  ring  grasp  the  outer  coating  of  the  charged  jar,  and 
let  the  last  boy  touch  the  knob  :  instantaneously  all  the  boys  in  the  ring 
will  receive  a  shock. 


ELECTRICAL    ATTRACTION    AND    REPULSION. 

17.  This  subject  has  been  fully  explained  in  the  prelimi- 
.nary  portion  of  this  work,  in  relation  to  a  numerous  class  of 
simple  experimental  facts.  But  the  electrical  machine  ena- 
bles us  to  exhibit  the  various  phenomena  of  electrical  attrac- 
tion and  repulsion  in  the  most  striking  manner. 

Exp.  1.  Repulsion  of  electrified  threads. — Take  a  skein  of  linen 
threads,  and,  after  tyigg  them  together  at  each  end,  suspend  them  from 
the  prime  conductor  of  the  machine.  When  the  handle  of  the  machine 
is  turned,  the  threads  will  become  electrified,  and  will  repel  each  other, 
so  that  they  will  swell  out  in  the  middle,  forming  a '  figure  resembling 
the  meridian  lines  on  a  globe. 

Exp.  2.  The  frightened  head  of  hair.  —  Fix  a  doll's  head  of  hair  in 
the  prime  conductor  ;  work  the  machine,  and  the  hairs  will  appear  to 
stand  on  end,  from  their  mutual  repulsion,  presenting  an  exaggerated 
appearance  of  a  person  in  a  state  of  fright. 


232 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


Present  a  pointed  rod  to  the  hairs,  and  they  will  immediately  col- 
lapse. 

A  bunch  of  large,  downy  feathers,  inserted  into  the  hole  of  the  prime 
conductor,  will  present  a  similar  appearance. 

Exp.  3.  The  electrical  dance.  —  In  this  experiment,  a  metal  plate  is 
suspended  by  a  chain  from  the  prime  conductor ;  a  few  inches  below  this 
plate  another  plate  is  placed  in  connection  with  the  earth ;  and  some 
light  figures  are  placed  upon  the  bottom  plate,  as  shown  in  Fig.  36. 
When  the  machine  is  worked,  the  figures  appear  to  dance,  or  to  jump  up 
and  down,  from  the  one  plate  to  the  other,  in  a  very  grotesque  manner. 


Fig.  36. 


Fig.  37. 


Exp.  4.  The  dancing  balls.  —  Here  a  number  of  cork  or  pith  balls  are 
placed  upon  a  metal  disk  P  (Fig.  37)  communicating  with  the  ground, 
and  the  whole  of  them  are  covered  with  the  glass  bell  B,  whose  upper 
part  is  open,  and  provided  with  a  collar  of  leather,  through  which  a  rod 
R  D  passes,  carrying  at  its  lower  extremity  a  metal  disk  D.  By  this 
construction,  the  upper  disk  D  can  be  placed  at  any  convenient  distance 
from  the  lower  disk  P.  The  ring  E,  of  the  rod  is  put  in  communication 
with  the  prime  conductor,  so  that,  when  the  machine  is  worked,  the  balls 
are  attracted  by  the  plate  D,  and  then  repelled  from  it,  being  charged 
with  positive  electricity  ;  now,  when  they  touch  the  bottom  plate  P,  the 
electricity  is  taken  from  them,  and  they  are  thus  prepared  to  be  again 
attracted  by  the  plate  D,  and  so  on. 

We  may  make  this  experiment  in  a  more  simple  manner  by  using  a 
glass  tumbler,  (Fig.  38,)  whose  interior  surface  has  been  electrified  by 
touching  its  different  parts  with  the  pointed  extremity  of  a  metal  rod 
fixed  in  the  conductor  of  an  electrical  machine  in  action.  The  glass  is 
then  inverted  upon  a  table,  over  a  lot  of  pith  balls ;  the  balls  immediately 


ELECTRICITY. 


233 


begin  to  dance,  being  alternately  attracted  and  repelled  by  the  electric, 
fluid  on  the  interior  surface  of  the  glass,  as  shown  in  Fig.  38. 


Fig.  38. 


Fig.  39. 


Exp.  5.  The  electrical  bells.  —  The  alternate  attraction  and  repulsion 
of  electrified  bodies  is  beautifully  illustrated  in  this  piece  of  apparatus, 
which  is  of  some  importance,  inasmuch  as  it  is  frequently  employed  in 
tropical  countries  to  detect  the  presence  of  an  electrified  cloud.  A  glass 
pillar  supports  two  metal  rods,  A  B  and  C  D,  from  which  four  bells, 
A'  B'  C'  D',  are  suspended  by  chains.  A  central  bell  G,  at  the  foot  of 
the  glass  pillar  E  F,  is  placed  on  the  wooden  stand  K ;  a  chain  G  K 
connects  this  bell  with  the  ground.  From  the  extremities  of  the  rods 
A  B  and  C  D,  four  small  brass  balls  H  H  are  suspended  by  silken 
threads.  When  the  machine  is  in  action,  the  cross  rods  are  put  in  con- 
nection with  the  prime  conductor,  and  the  four  bells  A'  B'  C'  D'  become 
charged  with  electricity,  and  consequently  attract  and  repel  the  insulat- 
ed balls  H  II.  When  the  balls  H  H  are  repelled,  they  strike  the  bell 
G,  to  which  they  give  up  the  electricity  they  received  from  the  electri- 
fied bells,  and  this  electricity  is  carried  off  by  the  chain  G  K.  The 
tinkling  noise  thus  produced  will  continue  so  long  as  electricity  is  sup- 
plied to  the  bells  A'  B'  C'  D  .  RP 

Fig.  40  represents  a  simpler  apparatus  of  this 
kind,  where  the  bells  are  hung  from  a  brass  rod  A  B, 
which  may  be  suspended  from  the  prime  conductor. 
In  this  form  of  the  apparatus,  the  central  bell  is 
suspended  by  a  silken  thread,  and  is  connected  with 
the  ground  by  means  of  the  chain  G  K. 

Exp.  6.  The  electrical  seesaiv.  —  This  consists  of 
a  small  strip  of  wood  (see  Fig.  41)  about  a  foot 
long,  covered  with  tin  foil,  and  insulated  on  c  like  a 
balance. 


Fig.  40. 


L*/ 

r 


234         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

.    A  slight  preponderance  is  given  to  it  on  the  side  a,  where  it  rests  on 
a  metal  ball  m  at  the  top  of  a  brass  wire  ;  p 
is  an  insulated  metal  ball.    The  ball  p  is  con- 
nected with  the  interior  coating  of  an  electri- 
cal jar,  while  m  is  connected  with  its  exterior 
coating.     When  the  jar  is  charged,  the  see- 
saw motion  will  immediately  be  produced.  Fl3'  **• 
The  cause  of  this  motion  depends  upon  the 
common  principle  of  electric  attraction  and  repulsion. 

This  experiment  will  succeed  quite  as  well  by  simply  connecting  the 
ball  p  with  the  prime  conductor  of  the  machine,  and  the  ball  m  with 
the  ground. 

Exp.  7.  The  electrified  water.  —  Here  a  little  metal  bucket  B,  having 
a  small  hole  in  its  bottom,  is  sus- 
pended from  the  prime  conductor  of 
the  electrical  machine.  The  hole  in 
the  bucket^  is  so  small  that  the  water 
merely  falls  from  it  in  drops  when  the 
machine  is  not  in  action ;  but  when 
the  machine  is  worked,  the  water  runs 
from  the  hole  in  a  continuous  stream, 
owing  to  the  repulsion  which  takes 
place  amongst  the  particles  of  the 
electrified  water. 

The  same  experiment  may  be  per-  ""** Fig  42 

formed   by  inserting  a  siphon  D  C, 
having  a  small  bore,  into  the  water,  as  shown  in  Fig.  42. 

A  similar  eifect  would  be  produced  by  suspending  a  sponge,  saturated 
with  water,  from  the  prime  conductor  of  the  machine. 

Exp.  8.  Electrified  sealing  wax.  —  Ignite  the  extremity  of  a  stick  of 
sealing  wax,  and  when  it  is  in  a  full  state  of  fusion,  blow  out  the  flame 
and  bring  the  melted  wax  near  to  the  prime  conductor  of  the  machine ; 
numerous  fine  filaments  of  wax  will  fly  to  the  conductor,  and  will  ad- 
here to  it,  forming  upon  it  a  sort  of  network  like  wool.  This  is  a  simple 
case  of  electrical  attraction.  The  experiment  will  succeed  best  if  a 
small  piece  of  wax  is  attached  to  the  end  of  a  metal  rod. 

Exp.  9.  The  electrical  swing  consists  of  a  light  figure  placed  upon  a 
swing  formed  by  a  silk  thread.  The  light  figure  swings  between  two 
balls,  one  of  which  is  insulated  and  put  in  connection  with  the  prime 
conductor,  the  other  ball  being  put  in  connection  with  the  ground.  The 
principle  of  this  apparatus  is  the  same  as  that  of  the  electrical  seesaw. 

Exp.  10.  The  electrical  swan. — In  this  experiment  a  light  piece  of 
cork,  or  any  other  light  substance,  cut  in  the  shape  of  a  swan,  is  made 


ELECTRICITY. 


235 


to  float  in  a  basin  of  water  placed  upon  the  insulated  stool.  The  water 
is  electrified  by  means  of  a  chain  which  passes  from  it  to  the  prime  con- 
ductor. The  little  floating  swan  will  approach  any  non- electrified  sub- 
stance that  may  be  presented  to  it. 

In  making  this  experiment,  the  cork  should  be  first 
completely  immersed  in  water,  to  render  it  a  con- 
ductor of  electricity. 

Exp.  11.  The  electrical  spider.  —  An  electrical  jar 
L  has  a  ball  b  connected  with  its  exterior  coating. 
When  the  jar  is  charged  with  the  positive  electricity 
of  the  prime  conductor,  any  light  substance,  such  as 
a  representation  of  a  spider,  suspended  between  the 
knobs  a  and  b,  will  oscillate  between  them. 


Fig.  43. 


LUMINOUS   EFFECTS   OF   ELECTRICITY. 


Fig.  44. 


THE    ELECTRIC    SPARK. 

18.  When  the  knuckle,  or  a  brass  ball  at  the  end  of  a  rod, 
is  presented  to  the  conductor  of  a  machine  in  full  action,  a 
spark  is  produced  by 
the  passage  of  the  fluid 
from  the  conductor  to 
the  knuckle.  The 
spark  has  a  zigzag 
form,  similar  to  a  flash 
of  forked  lightning.  The  length  and  intensity  of  the  spark 
depend  upon  the  power  of  the  machine.  Sparks  may  be 
taken  from  the  prime  conductor  of  a  very  powerful  machine 
at  the  distance  of  twenty  or  thirty  inches.  When  the  conti- 
nuity of  a  conducting  substance,  such  as  tin  foil,  is  broken  at 
different  parts,  a  spark  will  be  produced  at  every  place  where 
the  course  of  the  conductor  is  broken.  A  great  variety  of 
beautiful  experiments  may  be  made  to  illustrate  this  principle. 
These  experiments  should  be  made  in  the  dark. 

Exp.  1.  Luminous  spangles.  —  Sew  a  number  of  tin  foil  spangles  on 
silk  ribbon,  about  a  quarter  of  an  inch  apart ;  hold  the  ribbon  by  one 
extremity,  and  bring  the  other  near  to  the  prime  conductor ;  the  elec- 


236  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

tricity,  in  its  passage  from  spangle  to  spangle,  will  form  a  beautiful  line 
of  light. 

Exp.  2.    The  spiral  tube.  —  This  consists  of  two  glass  tubes,  about  a 
foot  long,  one  of  which  is  placed  within  the  other.     The  inner  tube  has 
spangles  of  tin  foil  pasted  on  its  outside  surface  in  the  form  of  a  spiral. 
The    two     ends     of     the    tubes    are 
mounted  with  brass  caps.     Hold  the 
tube  by  one  of  the  brass  caps ;  apply 
the  other  cap  to  the  prime  conductor  ;  Fig.  45. 

a   beautiful    spiral   stream  of   electric 
light  will  pass  from  one  end  of  the  tube  to  the  other. 

A  spiral  tube,  made  to  revolve  within  an  electrified  hoop  produces  a 
splendid  effect. 

Spangles  of  tin  foil  may  be  pasted  on  common  window  glass  so  as  to 
produce  various  luminous  devices,  such  as  geometrical  figures,  or  short 
words. 

Exp.  3.  Ignition  of  spirits  of  wine.  —  Let  a  person,  standing  on  the 
insulating  stool,  (see  Fig.  35,)  lay  one  hand  on  the  prime  conductor,  and 
with  the  other  hand  let  him  hold  a  warm  teaspoon  containing  spirits  of 
wine ;  let  some  other  person  present  his  knuckle  to  the  spoon,  and  the 
passage  of  the  spark  will  cause  the  spirits  to  ignite. 

Exp.  4.  Ignition  of  ether  on  water.  —  Pour  some  water  into  a  wine 
glass,  whose  outer  surface  is  perfectly  dry ;  pour  some  ether  on  the  top 
of  the  water,  and  connect  the  water,  by  means  of  a  chain,  with  the 
prime  conductor  of  the  machine.  Turn  the  handle  of  the  machine,  and 
present  your  knuckle,  or  a  metallic  ball,  to  the  surface  of  the  ether,  and 
the  electric  spark  will  ignite  the  ether. 

Exp.  5.    The  electrical  pistol.  —  The  electric  spark  will  readily  cause 
a  mixture  of  hydrogen  and  common  air  to  explode.     The  electrical  pis- 
tol,  represented  by  Fig.  46,  is   commonly 
employed  for  this  purpose  ;  a  is  a  brass  tube, 
or  barrel,  open  at  one  end ;  b  is  a  copper 
wire,  insulated  by  its  being  inserted  in  an 
ivory  tube,  which  passes  through  one  side  of 
the  barrel,  and  nearly  touches  the  inner  sur- 
face of  the  opposite  side.     Hold  the  mouth  Fig.  46. 
of  the  pistol  over  a  stream  of  hydrogen  gas, 

proceeding  from  a  pipe ;  after  a  sufficient  quantity  of  gas  has  entered, 
close  the  mouth  of  the  pistol  with  a  cork  c ;  take  a  spark  through  the 
knob  b,  and  the  cork  will  be  discharged  with  a  loud  report,  from  the 
explosion  of  the  gas  by  the  passage  of  the  spark  from  the  extremity  of 
the  wire  to  the  inner  surface  of  the  ban-el.  In  order  to  avoid  any  acci- 
dent, the  cork  should  be  attached  to  the  pistol  by  a  loose  string. 


ELECTRICITY.  237 

Exp.  6.  Ignition  of  common  gas.  —  Let  a  person,  standing  on  the 
insulated  stool,  touch  the  prime  conductor  with  one  hand,  and  with  the 
knuckle  of  the  fore  finger  of  the  other  hand  let  him  transmit  a  spark 
to  the  orifice  of  a  gas  pipe  from  which  a  current  of  gas  is  being  dis- 
charged, and  the  gas  will  be  ignited. 

Bring  a  candle  with  a  long  snuff,  that  has  just  been  extinguished,  near 
to  the  prime  conductor,  so  that  the  spark  passes  from  the  conductor, 
through  the  smoke,  to  the  candle  ;  it  is  relighted. 

DIFFERENT    FORMS    OF    THE    ELECTRIC    LIGHT. 

19.  The  intensity  of  the  electric  light  depends,  not  only 
upon  the  density  of  the  accumulated  electricity,  but  also  upon 
the  density  and  nature  of  the  gas  through  which  the  spark 
passes.  Thus  the  spark  is  bright  and  short  when  it  passes 
through  dense  air;  but  when  it  passes  tkrougli  rarefied  air  it 
is  long  and  dili'used,  and  of  a  violet  hue.  The  color  of  the 
spark  is  also  much  influenced  by  the  composition  of  the  gas 
through  which  it  is  transmitted,  as  well  as  by  the  nature  and 
form  of  the  conductor.  In  this  way  a  great  variety  of  sur- 
prising and  beautiful  luminous  experiments  may  be  performed. 

Exp.  1.  The  electric  light  from  points. — Place  a  pointed  rod  in  the 
prime  conductor  charged 
with  positive  electricity,  and 
the  electric  light  will  issue 
from  the  point  in  the  form 
of  a  brush.  Try  to  take  a 

spark  from  the  conductor,  Fig.  47. 

when  the  pointed  rod  is  at- 
tached to  it. 

Hold  the  point  of  the  rod  towards  the  prime  conductor,  and  a  star 
will  be  seen  on  the  point. 

Attach  the  pointed  rod  to  the  insulated  cushion,  charged  in  this  case 
with  negative  electricity,  and  the  electric  light  will  be  seen  in  the  form 
of  a  star. 

Insulate  the  cushion  as  well  as  the  prime  conductor,  and  attach 
pointed  rods  to  each  of  them,  so  that  the  points  may  be  at  the  distance 
of  four  or  five  inches  from  each  other ;  then,  upon  working  the  machine, 
a  brush  will  be  seen  upon  the  point  attached  to  the  prime  conductor, 
while  a  star  will  be  seen  upon  the  other  point,  presenting  the  appearance 
as  if  the  conductor  gave  out  its  electricity,  while  the  cushion  received  it. 


238          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

These  phenomena  were  at  one  time  considered  as  strong  arguments  in 
favor  of  Franklin's  theory  of  electricity. 

Exp.  2.  Passage  of  the  electric  light  through  rarefied  air.  —  Fix  a 
wire,  terminated  by  a  brass  ball,  to  the  plate  P  of  an  air  pump  ;  attach 
a  similar  ball  (by  a  sliding  wire  A  13)  to  the  top  of  the 
receiver  R,  so  as  to  bring  the  one  ball  over  the  other,  and 
at  the  distance  of  about  one  inch  apart.  Connect  the 
outer  ball  B  with  the  prime  conductor,  and  the  bottom 
plate  P  with  the  insulated  cushion.  Upon  turning  the 
handle  of  the  machine,  a  continuous  stream  of  electric 
light  will  pass  from  the  positive  to  the  negative  ball. 
While  no  light  is  exhibited  by  the  positive  ball,  a  beauti- 
ful luminous  atmosphere  entirely  surrounds  the  negative 
ball,  giving  the  appearance  of  a  fluid  in  the  act  of  pass- 
ing out  of  the  one  ball  and  entering  into  the  other.  By 
altering  the  distance  of  the  balls  from  each  other,  differ- 
ent  aspects  may  be  given  to  the  electrical  light. 

Exp.  3.  The  electrical  aurora  borealis.  —  Instead  of  the  receiver  ~R  of 
the  last  experiment,  let  a  glass  tube,  about  twenty  inches  long  and  three 
inches  in  diameter,  be  used ;  and  instead  of  the  two  discharging  balls, 
let  two  points  be  substituted.  When  the  tube  is  exhausted  of  air,  and 
the  machine  is  worked  in  the  dark,  the  whole  length  of  the  tube  will  be 
one  sheet  of  violet  red  light ;  if  a  small  portion  of  air  be  admitted,  nu- 
merous flashes  will  issue  from  the  points,  and  traverse  the  tube ;  when  a 
little  more  is  admitted,  these  flashes  will  appear  to  glide  in  a  serpentine 
manner  down  the  interior  of  the  tube.  The  succession  of  luminous 
phenomena,  in  fact,  bears  a  striking  resemblance  to  the  aurora  borealis. 

An  aurora  flask,  sold  by  instrument  makers,  answers  very  well  for 
exhibiting  these  phenomena. 

Exp.  4.  The  electric  spark  is  blue  when  transmitted  through  nitrogen. 
Exp.  5.  Passage  of  the  electric  light  through  the  Torricellian  vacuum. 
—  Seal  a  short  wire  within  one  end  of  a  glass  tube  about  32  inches  long ; 
attach  a  brass  ball  to  the  external  end  of  the  wire ;  fill  a  dry  tube  with 
mercury,  and  invert  it  in  a  cup  of  mercury  ;  a  vacuum  will  be  formed 
in  the  upper  part  of  the  tube ;  connect  the  ball  with  the  prime  con- 
ductor ;  turn  the  machine,  and  a  current  of  violet- colored  light  will  pass 
through  the  vacuum. 

MECHANICAL  EFFECTS  OF  ELECTRICAL  POINTS. 
20.    When  the  electric  fluid  discharges  itself  from  a  pointed 
conductor,  a  reaction  or  recoil  is  produced,  which  may  be 


ELECTRICITY. 


239 


used  to  give  motion  to  certain  delicate  pieces  of  mechanism, 
in  the  same  way  as  fluids  are  employed  in  the  common  re- 
action machines. 

Exp.  1.  The  electrical  wind.  —  Fix  a  pointed  rod  on  the  prime  con- 
ductor ;  work  the  machine ;  bring  the  back  of  your  hand  near  to  the 
point,  and  you  will  distinctly  feel  the  electrical  wind  proceeding  from 
the  point. 

Bring  the  flame  of  a  candle  near  to  the  point ;  the  flame  will  be  ex- 
tinguished by  the  electrical  wind,  chiefly  caused  by  the  repulsion  of  the 
electrified  air  from  the  point. 

Exp.  2.  The  electrical  fly  wheel. — A  metal  cross 
turns  on  a  pivot  which  is  fixed  on  the  prime  conduct- 
or ;  the  points  of  this  cross  are  all  bent  in  the  same 
direction ;  when  the  machine  is  turned,  the  fly  revolves 
in  the  directions  of  the  arrows  shown  in  the  figure ;  that 
is,  contrary  to  -the  direction,  in  which  the  points  are 
bent. 

The  fly  is  sometimes  mounted  on  an  insulated  stand,  as  shown  in 
Fig.  50. 

Exp.  3.  The  electrical  orrery.  —  This  instructive  and  elegant  piece  of 
apparatus  is  represented  by  Tig.  51 ;  where  S  represents  the  sun,  E 


Fig.  49. 


Fig.  51. 


Fig.  50. 


the  earth,  and  M  the  moon.  The  earth  and  the  moon  turn  upon  the 
pivot  B,  and  the  sun,  with  the  earth  and  the  moon,  turn  upon  the  pivot 
A,  which  is  placed  in  their  common  centre  of  gravity.  The  point  A  C 
is  fixed  on  the  prime  conductor.  The  points  a  and  G  are  so  placed  that 
all  the  pieces  revolve  in  the  same  direction  ;  that  is,  from  west  to  east. 

Exp.  4.  The  electrical  inclined  plane.  —  Here  the  recoil  of  the  elec- 
trical discharge  from  the  points  causes  the  fly  to  roll  up  an  inclined  plane 
formed  by  two  wires  A  B  and  C  D,  supported  by  insulating  pillars. 


240 


NATURAL    AND    EXPERIMENTAL   PHILOSOPHY. 


One  of  the  wires  is  connected  with 
the  prime  conductor  by  means  of  the 
chain  C  K. 

Exp.  5.  Repulsion  of  a  Point.  — 
Bring  an  insulated  point,  connected 
with  the  prime  conductor,  near  to 
the  electrical  swan,  (see  Exp.  10, 
p.  234;)  then,  instead  of  being  at- 
tracted, it  will  be  repelled.  This  is 


Fig.  52. 


caused  by  the  repulsion  of  the  electrified  air  from  the  point. 

On  this  principle,  a  light  paper  wheel  may  be  made  to  revolve  upon 
a  pointed  conductor  being  presented  to  its  sails. 

The  following  remarkable  experiment  depends  upon  the  same  prin- 
ciple :  — 

Pieces  of  phosphorus  are  put  into  the 
two  metal  cups  A  and  B  insulated  on 
glass  pillars;  a  candle  C  is  placed  ex- 
actly between  them ;  the  cup  A  is  con- 
nected with  the  prime  conductor,  and 
the  cup  B  with  the  insulated  cushion; 
when  the  machine  is  worked,  the  electric 
wind,  blowing  from  the  positive  cup  A  to 
the  negative  cup  B,  causes  the  flame  to  fly 
towards  the  cup  B,  and  to  heat  it,  so  as 
to  ignite  the  phosphorus. 

This  experiment  was  at  one  time  thought  to  be  a  decided  argument 
in  favor  of  the  single  fluid  theory ;  but  the  phenomenon  may  be  satisfac- 
torily explained  upon  the  theory  of  two  distinct  fluids. 


Fig.  53. 


PECULIAR   APPLICATIONS    OF    THE    PRINCIPLE    OP 
INDUCTION. 

21.  The  principle  of  induction  has  already  been  explained; 
but  the  following  experiment,  made  with  the  electrical  ma- 
chine, will  render  it  more  apparent. 

Exp.  1.  Take  an  insulated  metal  cylinder  B,  and  attach  small  pith 
balls,  suspended  from  cotton  threads,  to  different  parts  of  its  surface ; 
gradually  bring  an  electrified  body  A,  which  has  been  charged  with  the 
prime  conductor,  near  to  this  cylinder ;  when  A  is  about  an  inch  from 
the  conductor,  no  spark  having  passed  from  A  to  B,  the  pith  balls  at  tl.e 
extremities  C  and  D  diverge ;  at  E  and  F  the  divergence  is  less  than  it 


ELECTKICITY. 


241 


Fig.  54. 

is  at  C  and  D  ;  and  at  or  near  the  centre  Gr  the  balls  do  not  diverge 
at  all. 

As  we  have  already  explained,  the  positive  fluid  is  driven  to  the  ex- 
tremity C,  and  the  negative  fluid  is  drawn  to  the  extremity  D. 

When  A  is  withdrawn,  all  the  balls  fall  back  to  their  natural  position, 
and  the  positive  and  negative  fluids -on  the  conductor  B  reunite  and  re- 
turn to  their  natural  state  —  all  electricity  disappears. 

Before  withdrawing  A,  touch  the  extremity  C,  so  as  to  take  away  the 
positive  fluid,  and  the  conductor  will  remain  charged  with  negative  elec- 
tricity, and  so  on  as  described  in  the  experiments  given  at  page  222. 

Electricity  may  be  developed  by  induction  in  a  series  of 
insulated  conductors,  placed  in  a  line,  with  their  extremities 
in  order  near  to  each  other. 


The  Electrophones. 

22.   The  electrophorus,  invented  by  Volt  a,  depends  upon  the  principle 
of  induction  ;  it  is  capable  of  retaining  for  a  considerable  time  the  elec- 
tricity developed  upon  its  non-conducting  surface  by  friction.      It  is 
composed  of  a  cake  of  resin  poured  into  a  circular 
metal  mould  or  plate  b  5,  of  a  disk  of  metal  a  at 
a  little  less  than  the  cake,  furnished  with  an  in- 
sulating handle  g.    The  cake  of  resin  is  electrified 
negatively  by  rubbing  its  surface  with  a  cat's 
skin ;    the  metal  disk  is  then  placed  upon  the 
excited  cake ;  we  then  touch  the  plate- with  the 


Fig.  55. 


finger,  which  gives  us  a  spark  of  negative  electricity,  and  raise  it  by  the 
handle  g,  when  it  will  be  found  charged  with  positive  electricity  ;  upon 
touching  the  plate,  we  receive  a  spark  of  positive  electricity. 

When  we  first  touch  the  metal  plate,  (while  in  contact  with  the  res- 
in,) the  negative  electricity  is  taken  away  from  it,  owing  to  the  repul- 
sion of  the  negative  fluid  of  the  cake ;  now,  when  the  plate  is  raised  by 
21 


242          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

the  insulating  handle,  it  is  charged  with  positive  electricity,  because  the 
negative  fluid  had  been  taken  away  from  it,  while  the  positive  fluid  in  it 
remained  by  the  attraction  of  the  negative  fluid  of  the  cake. 

As  the  cake  will  retain  its  electricity  for  a  very  long  time,  any  num- 
ber of  sparks  may  be  taken  from  it  with  scarcely  any  diminution  of 
intensity. 

The  experiments  given  in  connection  with  Fig.  19,  page  222,  may  be 
explained  on  the  same  principle  as  that  of  the  electrophorus. 

Tate's  Electrophoric  Machines. 

23.  The  intensity  of  the  electricity  transmitted  to  the  con- 
ductor by  the  electrophorus,  described  at  page  220,  depends 
upon  the  following  circumstances  :  (1.)  The  size  of  the  plate  ; 
(2.)  The  completeness  of  the  contact  of  the  plate ;  (3.)  The 
rapidity  with  which  the  strokes  are  performed. 

The  following  contrivances  will  give  power  to  the  instru- 
ment, by  facilitating  the  operation,  and  by  lessening  the  time 
required  for  performing  each  stroke. 

Double-acting  Electrophorus •,  or  an  Electrophorus  capable  of 
producing  both  Kinds  of  Electricity. 

G  C          E          D 

24.  This  simple  contrivance  is  Q- 

represented  in  Fig.  56.  L  N  is 
an  open  box ;  L  N  sheet  gutta 
percha  stretched  tight  over  its 
top ;  J  K  the  plate  of  the  elec- 
trophorus ;  E  F  a  strip  of  double 
gutta  percha  attached  to  the  plate 


for  the  purpose  of  lifting  it,  form-  ^     /jg 

ing  a  loop  at  the  top  for  receiv- 
ing an  insulating  rod  D  C,  which  may  be  a  rod  of  glass  or  a  stick  of  seal- 
ing wax ;  G  C  H  a  bent,  insulated  wire,  terminated  with  knobs  G  and 
H ;  A  an  insulated  conductor  for  receiving  the  negative  electricity ;  B 
another  insulated  conductor,  for  receiving  the  positive  electricity  ;  these 
conductors  are  placed  at  the  distance  of  six  or  eight  inches  from  the 
plate  J  K,  and  the  length  of  the  wire  C  H  is  such  as  to  allow  the  knob  II 
to  come  into  contact  with  the  plate  J  K  at  the  same  time  as  the  knob 
G  comes  into  contact  with  the  conductor  A.  The  machine  is  worked  in 
the  following  manner  :  — 


ELECTRICITY. 


243 


Hub  the  surface  of  the  gutta  pcrcha  with  a  piece  of  fur  or  rabbit's 
skin  ;  place  the  plate  J  K  upon  the  excited  sheet,  taking  care  to  hold  it 
by  the  insulating  handle  C  D  ;  depress  the  handle  C  D,  until  the  knob 
H  comes  in  contact  with  the  plate  J  K  ;  then  a  spark  of  negative  elec- 
tricity will  be  transmitted  to  the  conductor  A ;  raise  the  plate  J  K  by 
means  of  the  insulated  handle  until  it  strikes  the  conductor  B  ;  then  a 
spark  of  positive  electricity  will  be  transmitted  to  the  conductor ;  and 
so  on  to  an  almost  indefinite  number  of  times.  The  action  of  the  ma- 
chine simply  consists  in  raising  and  depressing  the  hand. 

It  will  be  observed  that,  at  each  upward  stroke,  the  knob  G  is  raised 
from  the  conductor  A  before  the  plate  J  K  is  lifted  off  the  gutta  percha. 

The  conductors  A  and  B  may  be  used  in  the  same  way  as  the  con- 
ductors of  an  ordinary  electrical  machine  —  that  is,  for  charging  jars,  &c. 

Fig.  57  represents  another  form  of 
this  machine,  which  possesses  some 
advantages  over  that  just  described. 
J  K  represents  the  plate  ;  A  and  B 
the  conductors,  already  described; 
E  F  an  insulating  handle,  of  sealing 
wax,  or  glass  covered  with  sealing 
wax,  cemented  into  a  metal  tube 
F  D,  which  is  fixed  to  a  smaller  J 
tube  a  coming  in  contact,  time  after  ^' 

time,  with  the  plate  J  K ;  this  tube  a  works  smoothly  on  a  brass  rod  e 
fixed  to  the  plate  J  K,  having  a  stop,  or  small  rim,  at  its  top,  for  the 
purpose  of  stopping  the  ascent  of  the  small  tube  a  ;  F  G  is  a  wire  fixed 
to  the  tube  F  D,  and  terminated  by  a  knob  G.  By  this  contrivance  the 
rod  F  G  admits  of  an  up  and  down  motion  upon  the  pin  e,  at  the  same 
time  that  the  plate  J  K  admits  of  being  lifted  off  the  gutta  percha.  The 
machine  is  worked  in  the  following  manner  :  — 

Hold  the  plate  by  the  handle  E,  and  place  it  upon  the  excited  gutta 
percha  L  N,  (see  Fig.  56  ;)  depress  the  handle  E  until  the  knob  G  comes 
into  contact  with  the  conductor  A,  and  a  spark  of  negative  electricity 
will  be  transmitted  to  it ;  raise  the  handle  until  the  knob  G  comes  into 
contact  with  the  conductor  B,  and  a  spark  of  positive  electricity  will  be 
transmitted  to  it ;  and  so  on,  as  before  described. 


Single-acting  Electrophorus. 

25.  The  plates,  with  their  peculiar  appurtenances,  just  described,  may 
be  employed  with  great  advantage  in  the  place  of  the  simple  insulated 
plate  described  at  page  220.  The  contrivances  connected  with  these 
plates  enable  the  operator  to  perform  each  stroke  more  rapidly,  leaving, 


244 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


at  the  same  time,  his  left  hand  free  to  be  used  in  any  matter  requiring 
his  attention.  All  that  is  required  in  the  application  of  these  plates  to 
the  common  sheet  electrophorus  is  simply  to  have  a  conductor  placed  so 
as  to  come  in  contact  with  the  knob  .G  at  the  moment  the  plate  J  K 
falls  upon  the  excited  gutta  percha. 

Disguised  Electricity.     Condensers. 

26.  If  a  conductor  connected  with  the  ground  be  brought 
near  to  one  extremity  of  another  conductor  charged  with 
electricity,  then  the  quantity  of  the  electric  fluid  at  that  ex- 
tremity will  be  considerably  increased.  This  fact  is  just 
what  we  should  have  anticipated  from  the  peculiar  properties 
of  the  electric  fluid. 

Let  A  B  be  an  insulated  plate 
charged  with  electricity,  (say  with  -f- 
electricity ;)  A'  B'  another  plate,  con- 
nected with  the  ground  by  means  of 
the  chain  F'  G'.  Connect  A  B  with 
the  prime  conductor  by  means  of  the 
jointed  discharger  G  H  F ;  remove  the 
jointed  discharger :  then  A  B  will  be- 
come charged  with  positive  electricity, 
which  will  have  the  same  intensity  as 
that  of  the  prime  conductor ;  bring  the 
plate  A'  B'  near  to  the  charged  plate 
A  B  :  then  the  electricity  on  its  sur- 
face will  be  considerably  increased.  For  whilst  the  positive  electricity 
of  A  B  repels  the  positive  electricity  of  A'  B',  at  the  same  time  it  at- 
tracts its  negative  electricity;  but  this  negative  fluid,  accumulated  on 
the  plate  A'  B',  in  its  turn  reacts  upon  the  plate  A  B,  by  attracting 
more  of  the  positive  fluid  in  it  towards  the  surface  nearest  to  the  plate 
A'  B' ;  this  increase  of  fluid  on  the  plate  A  B  produces  a  further  action 
upon  the  plate  A'  B',  and  so  on  to  an  indefinite  scries  of  actions  and 
reactions.  The  negative  fluid  accumulated  in  A'  B'  is  called  disguised 
electricity,  for  it  cannot  be  detected  by  any  ordinary  means ;  it  is  re- 
tained or  held  there  entirely  by  the  attraction  of  the  positive  fluid  in 
A  B.  The  plate  A'  B'  is  called  the  condensing  plate,  and  A  B  the  col- 
lecting plate.  An  instrument  constructed  on  this  principle  is  called  the 
condenser. 

This  principle  of  disguised  electricity  may  be  readily  established  by 
experiment. 


ELECTRICITY. 


245 


I 


Fig.  59. 


Exp.  1.    Let  the  charged  plate 
A  B  be  connected  by  a  chain  with         ^ 
the  insulated  balls  F,  and  the  in-       j~]^~--*«cQi 
sulated  plate  A'  B'  with  the  insu-  rt^ 

lated  balls  F'.  First  charge  the 
plate  A  B,  (say  with  positive  elec- 
tricity :)  then  the  balls  F  will  di- 
verge ;  bring  the  plate  A'  B'  near 
to  A  B  :  then  the  electricity  in 
A'  B'  will  be  decomposed,  and  the 
balls  will  diverge.  Touch  A'  B'  with  the  finger  so  as  to  carry  away  its 
positive  electricity  set  free :  then  the  balls  F'  will  immediately  cease  to 
diverge,  and  the  balls  F  will  have  now  only  a  very  feeble  divergence. 
The  negative  electricity  in  A'  B'  exists  in  a  disguised  state.  Withdraw 
A  B  and  A'  B'  from  each  other,  taking  care  not  to  touch  them :  then 
immediately  the  balls  diverge  —  those  at  F  with  positive  electricity,  and 
those  at  F'  with  negative.  Bring  the  plates  again  near  to  each  other, 
and  the  divergence  of  the  balls  F'  again  ceases,  and  that  of  F  diminishes. 
The  negative  fluid  of  the  plate  A'  B'  is  again  disguised,  and  the  positive 
fluid  is  partly  withdrawn  from  the  extremity  F  towards  the  extremity 
A  B  by  the  attraction  of  the  negative  fluid  in  the  plate  A'  B'. 

These  facts  enable  us  to  give  a  satisfactory  explanation  of 
the  principle  of  the  condenser,  of  the  electroscope,  and  of  the 
Ley  den  jar. 

The  Condenser. 

27.  The  condenser,  the  principle  of  which  has  just  been 
explained,  is  used  to  detect  the  presence  of  electricity  where 
it  is  so  very  "small  as  to  require  it  to  be  collected  and  con- 
densed before  it  will  affect  the  electroscope. 

It  consists  of  two  disks  of  metal  b  b  and  c  c,  whose 
touching  surfaces  are  polished  and  covered  over  with 
a  thin  coat  of  varnish  or  some  non-conducting  sub- 
stance ;  the  upper  plate  is  the  collector,  and  the 
lower  one  the  condenser;  the  condenser  stands  on 
an  insulating  glass  pillar  n,  and  the  collector  has  an 
insulating  handle  m  attached  to  it,  by  which  it  may 
be  lifted  ;  a  brass  wire  a  b  with  a  knob  a  is  fixed  to 
the  under  side  of  the  condensing  plate,  for  the  purpose  of  connecting  it 
with  the  ground. 

21* 


Fig.  60. 


246          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


The  apparatus  is  thus  used  :  Place  the  body  whose  electricity  is  to  "be 
examined  in  connection  with  the  collector  c  c  ;  touch  the  ball  a  with 
the  finger,  and  after  having  taken  it  away,  suddenly  raise  the  collector 
by  the  glass  handle  m,  and  the  electricity  of  the  body  under  examina- 
tion will  have  accumulated  itself  in  the  collector,  and  the  -opposite  fluid 
will  be  found  in  the  condenser ;  present  the  collector  to  any  delicate 
electroscope  or  electrometer,  and  the  accumulated  electricity  will  be 
rendered  apparent.  The  rationale  of  this  process  has  already  been  ex- 
plained. 

28.     ELECTROSCOPES    AND    ELECTROMETERS. 

There  are  a  great  variety  of  electroscopes.  For  all  ordinary  purposes, 
the  pith  ball  electroscope,  represented  in  Fig.  3,  or  that  described  at  page 
214,  is  quite  sufficient.  But  in  pursuing  many  electrical  inquiries,  we 
require  instruments  of  more  delicacy,  or  of  more  durability. 

In  order  to  render  electroscopic  instruments  more  sensitive  and  more 
accurate,  the  two  light  bodies  are  suspended  from  a  metal  rod  and  en- 
closed in  a  glass  bell,  and  the  extremity  of  the  rod  (which  is  either  a 
knob  or  a  plate)  is  to  be  touched  with  the  electrified  substance.  The 
light  bodies,  thus  suspended,  are  either  pith  balls,  as  shown  in  Fig.  61, 


Fig.  61. 


-Fig.  62. 


Fig.  63. 


or  two  gold  leaves,  as  in  Bennet's  electrometer,  or  the  gold  leaf  electrom- 
eter, shown  in  Figs.  62  and  63.  In  Fig.  62,  two  knobs  A'  and  B'  are 
placed  on  each  side  of  the  gold  leaves  f  f,  so  that  when  the  leaves  di- 
verge too  strongly,  they  impinge  upon  the  knobs,  and  are  thus  dis- 
charged of  their  electricity;  this  contrivance  prevents  the  leaves  from 
being  torn  by  adhering  to  the  sides  of  the  glass  bell. 

In  order  to  insulate  the  electricity  given  to  the  cap  or  plate  K,  the 
metal  rod  carrying  the  gold  leaves  passes  through  a  glass  tube,  which  is 
cemented  to  a  ferrule  on  the  plate  A  B*  closing  the  top  of  the  glass 


ELECTRICITY.  247 

cover.  (See  Fig.  63.)  This  plate  is  screwed  upon  the  glass  cover,  so  that 
the  leaves  may  be  placed  within  the  glass  without  injuring  them.  The 
gold  leaves  are  attached  to  the  lower  extremity  of  the  metal  rod,  simply 
by  the  adhesion  of  gum.  Before  using  any  electrometer,  it  is  important 
that  all  its  parts  be  perfectly  dry,  and  that  the  surrounding  air  be  warm 
and  free  from  moisture. 

To  use  the  gold  leaf  electroscope :  Bring  an  excited  glass  tube  near  to 
the  cap  K,  and  the  gold  leaves  will  diverge  with  positive  electricity, 
because  the  positive  fluid  of  the  glass  drives  the  positive  fluid  of  the  cap 
into  the  gold  leaves.  Excited  sealing  wax  brought  near  to  the  cap  will 
cause  the  leaves  to  collapse. 

The  following  is  the  best  method  of  using  the  simple  gold  leaf  elec- 
trometer represented  in  Figs.  62  and  63  ;  for  it  causes  the  gold  leaves  to 
be  permanently  divergent.  Electrify  a  stick  of  sealing  wax  ;  hold  the 
electrified  wax  very  near  to  the  cap  K,  without  touching  it ;  the  gold 
leaves  will  diverge  from  each  other  on  the  principle  of  induction,  with 
the  same  electricity  as  the  wax,  that  is,  with  negative  electricity ;  touch 
the  cap  with  the  finger,  and  the  gold  leaves  instantly  collapse ;  first  re- 
move the  finger,  then  the  electrified  body  and  the  gold  leaves  will 
remain  permanently  divergent,  with  an  electricity  opposite  to  that  of  the 
wax ;  that  is,  with  positive  electricity.  Now  bring  an  electrified  glass 
tube  near  to  the  cap  K,  and  the  divergence  of  the  leaves  will  be  in- 
creased, because  the  glass,  being  positive,  will  drive  more  of  the  positive 
fluid  into  the  gold  leaves.  After  taking  the  glass  rod  away,  bring  elec- 
trified brown  paper  near  the  cap  of  the  electroscope ;  the  divergence  of 
the  gold  leaves  will  be  decreased,  because  the  brown  paper,  being  nega- 
tive, will  drive  the  negative  fluid  into  the  gold  leaves,  thereby  neutral- 
izing the  positive  fluid  at  first  in  them. 

It  should  be  observed  that  where  the  charge  of  the  leaves  is  temporary, 
the  electricity  is  the  same  as  the  excited  body ;  but  where  the  charge  is 
permanent,  as  in  the  preceding  case,  the  electricity  is  of  an  opposite  kind. 

Experiments  with  the   Gold-leaf  Electroscope. 

Exp.  1.  Strike  the  cap  of  the  electroscope  with  a  warm  silk  handker- 
chief; the  leaves  will  diverge  with  negative  electricity.  Verify  this  by 
bringing  an  excited  stick  of  sealing  wax  near  to  the  cap. 

Exp.  2.  Excite  a  silk  ribbon  ;  bring  it  near  to  the  cap  of  the  electro- 
scope ;  the  leaves  instantly  diverge :  excite  a  glass  rod ;  bring  it  also 
near  to  the  cap ;  the  divergence  of  the  leaves  is  diminished,  thereby 
showing  that  the  electricity  of  silk  is  negative. 

Exp.  3.  Rub  a  toll  of  brimstone  with  a  piece  of  warm  flannel,  hold 
the  excited  brimstone  near  to  the  cap  of  the  electroscope,  touch  the  cap 


248 


NATURAL    AND    EXPERIMENTAL    PHIL'OSOPHY. 


with  the  finger ;  first  take  away  the  finger,  and  then  the  brimstone ;  the 
gold  leaves  will  remain  permanently  divergent  with  positive  electricity. 
Verify  this  by  bringing  an  excited  stick  of  sealing  wax  near  to  the  cap. 

Exp.  4.  Place  a  tin  vessel  containing  water  on  the  cap  K  of  the  elec- 
troscope, (see  Fig.  63  ;)  drop  a  red  hot  cinder  into  the  water  ;  the  leaves 
will  instantly  diverge."  Here  the  escape  of  steam  generates  electricity. 

The  gold-leaf  condensing  electroscope,  represented  in  Pig.  64,  simply 
consists  in  the  application  of  the  condenser,  de- 
scribed at  page  240,  to  the  gold  leaf  electroscope. 
Here  c  c'  is  the  collecting  plate,  with  its  glass 
handle  m,  placed  upon  the  plate  of  the  electro- 
scope. In  order  to  render  this  instrument  more 
delicate,  the  glass  bell  of  the  ordinary  electro- 
scope is  enclosed  by  a  glass  case,  into  which 
some  chloride  of  calcium  is  put,  with  the  view 
of  absorbing  any  moisture  which  may  be  in  the 
circumjacent  air. 

The  degree  of  divergence  of  the  gold 
leaves  only  gives  us  a  rude  idea  of  the 
intensity  of  the  electricity  with  which 
an  excited  body  is  charged ;  for  the  di- 
vergence is  not  exactly  in  proportion  to 
the  intensity  of  the  charge.  These  in- 
struments, therefore,  should  be  called 

electroscopes  rather  than  electrometers.  The  name  of  elec- 
trometer should  only  be  given  to  such  instruments  as  Cou- 
lomb's balance,  which  afford  us  the  means  of  exactly  com- 
paring the  electrical  intensities  of  any  two  bodies. 

The  needle  electrometer,  represented  in  Fig.  65,  is  a  rod 
or  needle  balanced  on  a  point,  having  pith  balls  fixed  to 
its  extremities. 

Coulomb's  torsion  electrometer.  —  For  ordinary  pur- 
poses, the  instrument  represented  in  Fig.  66  will  be 
found  exceedingly  useful.  A  small  disk  of  gilt  paper  C 
is  attached  to  the  end  of  a  needle  of  gum  lac  or  sealing  wax ;  the  needle 
is  suspended  by  a  thread  of  sealing  wax  K  D,  after  the  manner  de- 
scribed at  page  214,  and  placed  within  a  glass  jar  or  bottle,  as  shown  in 
the  figure  ;  passing  through  the  side  of  the  jar,  and  on  a  level  with  the 
needle,  is  a  brass  wire,  terminated  with  gilt  balls  A  and  B.  To  use  the 
instrument,  turn  the  knob  K,  if  necessary,  so  as  to  bring  the  disk  C  in 


Fig.  65. 


ELECTRICITY. 


249 


Fig.  66. 


contact  with  the  ball  B  ;  touch  the  ball  A  with  the  elec- 
trilied  body ;  then  C,  being  electrified  in  the  same  way 
as  B,  will  be  repelled,  and  the  angle  of  torsion,  or  twist, 
will  indicate  the  force  of  repulsion,  or,  what  is  the  same 
thing,  the  relative  amount  of  electrical  charge  given  to 
A,  It  will  be  observed  that  the  force  requisite  to  twist 
a  thread  is  in  proportion  to  the  angle  over  which  the 
needle  is  moved,  so  that  the  angle  of  deflection  is  a  true 
measure  of  the  electrical  repulsion. 

In  comparing  the  intensity  of  two  electrified  surfaces,  it  is  necessary 
that  we  should  employ  a  proof  plane,  (which  is  a  round  piece  of  gilt 
paper  fixed  to  the  end  of  a  rod  of  sealing  wax  or  shell  lac,)  for  the  pur- 
pose of  transferring  the  charges  of  electricity  from  the  electrified  surface 
to  the  ball  A  of  the  electrometer. 

It  is  obvious  that  the  torsion  electrometer  may  be  used,  like  the  gold 
leaf  electroscope,  for  ascertaining  whether  a  body  is  positively  or  nega- 
tively electritied. 

Pig.  67  represents  the  form  usually  given  to  the 
torsion  electrometer,  where  the  thread  b  B,  support- 
ing the  needle  b  d,  passes  through  a  tube  mounted 
on  the  glass  jar  A.  The  circumference  of  the  jar 
is  divided  into  degrees,  the  zero  point  being  oppo- 
site to  the  ball  to  which  the  electricity  is  trans- 
ferred, so  that  the  angle  through  which  the  needle 
is  repelled  may  be  at  once  seen.  The  needle  is 
usually  supported  by  a  fine  thread  of  silver,  about 
two  feet  long,  fixed  at  the  top  of  the  tube  to  a 
brass  piece  c,  which  admits  of  being  turned  tightly 
round  the  cap,  Avhich  is  also  of  brass,  and  fixed  to 
the  tube  itself. 

By  means  of  the  torsion  electrometer, 
Coulomb  proved  that  the  law  of  electrical  „. 

attraction  and  repulsion,  as  influenced  by 
distance,  is  the  same  as  the  law  of  gravitation ;  that  is,  in- 
versely as  the  square  of  the  distance.     He  also  determined 
the  law  regulating  the  distribution  of  the  electric  fluid  on  the 
surfaces  of  conductors. 


250  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


THE  LEYDEN  JAR  AND  ELECTRICAL  BATTERY. 

29.     EXPERIMENTS    WITH    A    SINGLE    LEYDEN    JAR. 

Exp.  1.  To  give  an  electrical  shock.  —  Charge  the  jar  after  the  manner 
described  at  page  228  ;  grasp  the  outside  of  the  jar  with  one  hand,  and 
touch  the  knob  of  the  jar  with  the  other  hand,  and  an  electric  shock  will 
be  felt.  Care  should  be  taken  that  the  jar  is  not  too  strongly  charged. 
Generally  speaking,  about  half  a  dozen  good  sparks,  transmitted  to  the 
knob  of  the  jar,  will  be  a  sufficient  charge  for  giving  any  person  a  shock. 

A  shock  may  be  given  to  any  number  of  persons  at  the  same  time. 
Let  them  form  themselves  into  a  ring,  by  taking  hold  of  each  other's 
hands ;  let  the  first  person  grasp  the  outside  coating  of  a  jar  which  has 
been  charged,  and  then  let  the  last  person  in  the  ring  touch  the  knob  of 
the  jar  ;  the  whole  of  the  persons  forming  the  ring  will  instantaneously 
receive  the  shock.  The  number  of  the  persons  forming  the  ring  does 
not  appear  to  affect  the  intensity  of  the  shock. 

Exp.  2.  To  show  the  striking  distance  of  the  spark  at  discharge. — 
Touch  the  outside  coating  of  a  charged  jar  with  one  ball  of  the  jointed 
discharging  rod ;  gradually  bring  the  other  ball  towards  the  knob  of  the 
jar ;  then,  when  they  have  come  sufficiently  near  to  each  other,  the  elec- 
tric spark  will  pass  from  one  ball  to  the  other  with  a  snapping  noise. 
The  distance  at  which  the  discharge  takes  place  depends  upon  the  size 
of  the  jar  and  the  intensity  of  the  charge. 

Exp.  3.  To  show  the  mariner  in  which  a  jar  becomes  charged.  —  Place 
a  common  Leyden  jar  upon  the  insulated  stool,  and  bring  the  knob  within 
striking  distance  of  the  prime  conductor ;  turn  the  machine,  and  it  will 
be  found  that  the  jar  cannot  be  charged  when  its  outside  coating  is  thus 
insulated  :  now  bring  your  knuckle  near  the  outside  coating  of  the  jar ; 
then,  for  every  spark  of  positive  electricity  which  passes  to  the  interior 
coating  of  the  jar,  a  corresponding  spark  of  positive  electricity  will  pass 
from  the  outside  coating  to  the  knuckle.  The  positive  electricity  is 
driven  off  from  the  outside  coating  on  the  principle  of  induction,  while 
the  negative  electricity  is  held  in  a  disguised  condition  on  the  outside 
coating  by  the  attraction  of  the  positive  electricity  accumulated  on  the 
inside  coating.  Hence  it  appears,  that  when  the  inside  coating  is 
charged  positively,  the  outside  coating  is  charged  negatively ;  and  that 
when  the  jar  is  being  discharged,  the  two  opposite  fluids  rush  to  each 
other. 

Exp.  4.  To  charge  the  inside  of  ajar  negatively.  —  Place  the  jar  upon 
the  insulated  stool ;  bring  the  outside  coating  of  the  jar  within  the 
striking  distance  of  the  spark  of  the  prime  conductor ;  turn  the  machine, 


ELECTRICITY. 


251 


and  at  the  same  time,  apply  the  knuckle  to  the  knob  of  the  jar  ;  then, 
for  every  spark  of  positive  electricity  which  passes  to  the  outside  coating, 
a  corresponding  spark  of  positive  electricity  passes  from  the  inside  coat- 
ing to  the  knuckle,  and  thus  the  jar  will  become  charged  with  negative 
electricity. 

Exp.  5.  To  show  the  principle  of  disguised  electricity  in  relation  to  the 
Leyden  jar.  —  Let  a  jar  be  placed  on  the  insulating  stool,  and  let  the 
ball  D',  supported  by  a  metal  pillar,  communicate  with  the  outer  coating 
of  the  jar.  Suspend  a  ball  of  cork  F,  by  a  linen  thread,  midway  be- 
tween the  knob  D  of  the  jar  and  the  baU  D?,  communicating  with  the 
ground  by  a  metal  chain  K.  Charge  the  jar  after  the  manner  described 
in  Exp.  3  ;  then  the  ball  will  be 
attracted  to  D,  and,  owing  to  the 
contact,  a  certain  portion  of  posi- 
tive electricity  will  pass  to  the 
ground  through  K,  and  a  certain 
portion  of  positive  electricity  will 
remain  disguised  on  the  inner 
coating ;  F,  being  thus  restored  to 
its  natural  state,  will  be  attracted 
to  the  ball  D',  owing  to  the  nega- 
tive electricity  set  free  from  the 
external  surface  of  the  jar :  when 
F  comes  in  contact  with  D',  a 
certain  portion  of  electricity  mil, 


Fig.  68. 


in  like  manner,  pass  off  from  the  outer  surface  of  the  jar  through  the 
conductor  K,  and  then  a  certain  portion  of  negative  electricity  will  re- 
main disguised  on  the  outer  coating ;  F  will  then  be  again  attracted  to 
D ;  and  so  on.  The  ball  F  may  continue  to  oscillate  betw.een  the  two 
knobs  D  and  D'  for  several  hours ;  at  the  end  of  which  time  the  two 
coatings  will  have  lost  their  electricity  by  this  succession  of  small  dis- 
charges. 

The  apparatus  represented  in  Fig. 
69  is  intended  to  illustrate  the  same 
principle.  The  insulated  balls  on  F 
are  in  connection  with  the  inner 
coating,  and  those  on  F;  are  in  con- 
nection with  the  outer  coating. 
Charge  the  jar  after  the  manner  de- 
scribed in  Exp.  3  ;  then  the  balls  F 
will  diverge  with  positive  electricity, 
and  the  negative  electricity  will  be  held  in  a  disguised  state  on  the  outer 
coating.  Touch  the  knob  D,  and  the  balls  F  will  collapse,  while  the 


Fig.  69. 


252          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

balls  F'  will  diverge  ;  the  positive  electricity  is  now  in  a  disguised  state, 
while  the  negative  is  free ;  and  so  on,  until  all  the  fluid  is  taken  from 
the  jar. 

Exp.  6.  To  make  a  jar  out  of  a  common  vial.  —  Fit  a  cork  to  the 
vial,  and  pass  a  wire  through  it,  reaching  nearly  to  the  bottom  of  the 
vial ;  put  a  knob  on  the  outer  extremity  of  the  wire ;  half  fill  the  vial 
with  water,  and,  after  carefully  drying  the  outside,  put  the  cork  with 
its  wire  in  its  place  ;  grasp  the  outside  of  the  vial  with  one  hand,  and, 
after  having  taken  a  few  sparks  from  the  prime  conductor  to  the  knob, 
touch  the  knob  with  the  other  hand,  and  you  will  receive  an  electric 
shock. 

Here  the  hand  answers  the  purpose  of  the  external  coating  of  the 
Leyden  jar,  and  the  water  that  of  the  internal  coating. 

Exp.  7.  The  electrical 
sportsman.  —  This  consists  of 
a  jar  J  connected  with  the 
figure  D  of  a  sportsman,  who 
is  supposed  to  be  in  the  act  of 
shooting  some  birds  flying 
over  the  ball  A.  The  knobs 
A  and  B  are  connected  with 
the  inner  coating  of  the  jar, 
and  the  knob  C  at  the  ex-  .  Fig.  70. 

tremity  of   the    sportsman's 

gun  is  connected  by  a  wire  going  down  the  figure  with  the  outer  coating. 
The  figure  admits  of  being  turned  round  upon  a  pin  D  at  its  foot.  Some 
light  substances,  cut  in  the  shape  of  birds,  are  suspended  by  cotton 
threads  from  the  ball  A.  Charge  the  jar ;  the  birds  appear  to  fly,  owing 
to  their  mutual  repulsion ;  turn  the  sportsman  round  until  you  bring  the 
muzzle  C  of  his  gun  within  striking  distance  of  the  spark ;  at  the  mo- 
ment the  snap  and  spark  of  discharge  takes  place,  the  pith  birds  appear 
to  fall  down  as  if  they  were  shot. 

Exp.  8.  To  ignite  cotton.  —  Tie  a  bit  of  cotton,  mixed  with  a  little 
powdered  resin,  on  one  of  the  knobs  of  the  jointed  discharger ;  place  the 
other  knob  in  contact  with  the  outer  coating  of  a  charged  jar ;  bring  the 
knob,  covered  with  the  cotton,  within  striking  distance  of  the  knob  of 
the  jar  :  and  the  spark  will  ignite  the  cotton. 

Exp.  9.  To  perforate  a  card. —  Hold  a  dry  piece  of  card  paper  in  con- 
tact with  one  of  the  knobs  of  the  jointed  discharger  ;  discharge  the  jar 
through  the  card  paper,  and  it  will  be  found  to  be  perforated  by  the  pas- 
sage of  the  spark. 

Discharge  the  jar  through  three  or  four  pieces  of  card  paper,  or  through 
about  a  dozen  sheets  of  writing  paper. 


ELECTRICITY. 


253 


The  hole  in  the  paper  will  be  always  found  to  be  burred  equally  on 
each  side,  as  if  the  electric  fluid  had  come  from  the  middle  of  the  card. 

Exp.  10.  The  magic  picture.  —  This  is  simply  a  pane  of  glass  placed 
in  a  frame,  and  covered  on  both  sides  with  tin  foil  within  a  few  inches 
of  the  edges.  It  answers  the  same  purpose  as  the  Leyden  jar.  Charge 
one  side  of  the  plate  after  the  manner  described  in  Exp.  3  ;  discharge 
the  plate  in  the  usual  way. 


Fig.  71. 


Fig.  72. 


Exp.  11.  The  electric  pendulum.  —  Make  an  electric  pendulum  of 
wire,  with  pith  balls  at  the  end  of  it,  as  represented  in  Fig.  72.  Bal- 
ance the  pendulum  on  the  edge  of  a  charged  plate  of  glass;  the  pendu- 
lum will  vibrate ;  the  balls  alternately  strike  the  plate. 


ELECTRICAL    BATTERIES. 

30.  An  electrical  battery  is  formed  when  several  jars  are 
united  together,  by  establishing  a 
metallic  connection  between  all 
their  inner  coatings,  and  a  simi- 
lar connection  between  all  their 
outer  coatings.  The  jars  are 
placed  in  a  wooden  box  lined 
with  tin  foil,  upon  which  the  jars 
stand,  and  which  forms  the  con- 
nection between  all  the  outer 
coatings ;  the  inner  coatings  com- 
municate togethei;by  means  of  metal  rods,  which  connect  the 
22 


254: 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


various  knobs  of  the  jars  together.  The  battery  is  usually 
discharged  by  means  of  a  chain,  which  has  one  of  its  extrem- 
ities fixed  to  the  tin  foil  of  the  case,  and  the  other  extremity 
attached  to  the  knob  of  a  discharging  rod. 

It  always  requires  time,  even  with  a  good  machine,  to  charge  a  large 
battery.  In  order  to  accelerate  the  operation,  a  peculiar  contrivance, 
represented  by  Fig.  74, 
has  been  adopted,  called  '••D' 
charging  by  cascade.  Here 
each  jar  of  the  battery  is 
placed  upon  an  insulating 
stool,  and  the  knob  of 
each  is  connected  by 
means  of  a  chain  C  with 
the  outer  coating  of  the  preceding  one  ;  the  knob  D'  of  the  first  jar  A1 
is  connected  with  the  prime  conductor,  and  the  outer  coating  of  the  last 
A4  is  connected  with  the  ground  by  means  of  the  chain  D.  When  the 
machine  is  worked,  the  positive  electricity  from  the  outer  coating  of  A1, 
in  place  of  being  driven  away  into  the  ground,  serves  to  charge  A2,  by 
passing  into  its  inner  coating ;  in  like  manner,  the  positive  electricity 
driven  off  from  the  outer  coating  of  A2  serves  to  charge  A3 ;  and  so  on, 
until  the  positive  electricity  is  carried  away  from  the  outer  coating  of  the 
last  jar  into  the  ground  by  means  of  the  chain  D. 

The  battery  is  discharged  by  connecting  D  with  D'. 


DISCHARGING    ELECTROMETERS. 

31.  In  these  electrometers,  the  intensity  of  the  electricity 
is  measured  by  the  length  of  the  spark  at  the  instant  of 
discharge. 

Lane's  discharging  electrometer.  —  This  is  an  ordi- 
nary Leyden  jar,  having  an  arm  c  d  e  attached  to 
the  conducting  wire  a  b ;  the  horizontal  part  c  d  is  of 
glass,  coated  over  with  shell-lac  ;  the  vertical  part 
d  e  is  a  brass  rod,  having  a  ring  e,  in  which  the 
graduated  wire  m  o  slides,  and  terminating  in  a  knob 
o;  the  distance  between  the  knobs  o  and  6,  and 
consequently  the  length  of  the  spark,  can  thus  be 
measured.  To  use  the  jar,  connect  the  extremity  m 
of  the  sliding  wire,  by  means  of  a  chain,  with  the  Fig.  75. 


ELECTRICITY. 


255 


outer  coating  of  the  jar,  and  then  adjust  the  distance  between  the  knobs 
o  and  b  to  suit  the  amount  of  charge  which  you  wish  to  give  to  the  jar. 
Bring  the  knob  b  near  to  the  prime  conductor,  and  continue  to  work 
the  machine  until  the  discharge  takes  place  between  the  knobs  b  and  o. 
If  the  knobs  b  and  o  are  placed  very  near  together,  the  intervening 
space  will  be  penetrated  by  the  spark  when  only  a  small  charge  has 
been  given  to  the  jar ;  but  if  the  distance  between  them  be  in- 
creased, then  a  more  powerful  charge  may  be  given  before  the  spon- 
taneous discharge  takes  place.  If  the  same  distance  between  the  balls 
o  and  b  be  retained,  then  the  discharge  will  always  take  place  when  the 
same  quantity  of  electricity  has  been  transmitted  to  the  jar.  This  jar 
may  be  used  to  test  the  relative  powers  of  two  electrical  machines ;  in 
order  to  do  this,  you  place  the  balls  o  and  b  at  a  certain  convenient  dis- 
tance from  each  other  :  then  that  machine  will  be  most  powerful  which, 
causes  the  jar  to  be  discharged  with  the  least  number  of  turns  of  the 
handle. 

Cuthbertson' s  discharging  electrome- 
ters. —  This  apparatus,  represented  in 
Fig.  76,  effects  the  discharge  of  itself 
when  the  jar  or  battery  has  arrived  at 
the  limit  of  its  charge. 

An  insulating  support  A  B  carries  a 
metal  rod  D  C,  turning  on  a  centre  at 
B  like  the  two  arms  of  a  balance.  This 
metal  rod  is  connected  with  the  inner 
coating  of  the  jar,  or  battery,  and  also 
with  a  quadrant  electrometer,  as  shown 
in  the  figure.  Below  the  knob  C,  at  a 
sufficient  distance  to  prevent  discharge, 


Fig.  76. 


is  another  knob  E,  which  communicates  with  the  outer  coating  by  means 
of  the  chain  F  ;  nearly  in  contact  with  the  knob  D  is  another  knob  D', 
placed  at  the  extremity  of  a  metal  rod,  which  is  fixed  to  the  same  sup- 
port as  the  rod  D  C,  and,  being  in  metallic  communication  with  it,  is 
also  connected  with  the  inner  coating  of  the  jar  or  battery.  When  the 
jar  has  become  sufficiently  charged,  the  knob  I)  is  repelled  from  the  knob 
D',  and  the  knob  C  is  thereby  brought  nearer  to  the  knob  E  in  connec- 
tion with  the  outer  coating ;  and  when  this  distance  is  within  the  dis- 
tance at  which  explosion  takes  place,  the  jar  or  battery  is  discharged. 

Fig.  77  represents  a  slightly  different  form  of  this  apparatus,  where 
L  is  a  sliding  ball,  which  enables  the  operator  to  give  a  more  perfect 
adjustment  to  the  action  of  the  apparatus. 

The  balance  electrometer  simply  consists  of  a  common  balance  beam, 
with  a  scale  hung  on  one  side  for  holding  weights,  and  a  gilt  piece 


256 


NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 


of  wood  hung  on  the  other  for  the 
purpose  of  being  applied  to  the  surface 
of  an  electrified  body.  The  weight  ne- 
cessary for  overcoming  the  attraction  of 
the  electrified  surface  on  the  gilt  piece 
of  wood  is  taken  as  the  relative  measure 
of  the  intensity  of  the  electricity  on  the 
surface  of  the  electrified  body. 


Fig.  77. 


MECHANICAL    EFFECTS    OF   ELECTRIC    DISCHARGES. 

32.  The  following  experiments  may  be  performed  with  a  single  jar  ; 
but  the  effects,  in  most  cases,  will  be  more  striking  when  a  batteiy  is 
used. 

Exp.  1.  The  thunder  house.  —  This  apparatus  illus- 
trates the  use  of  metallic  rods  as  a  protection  to  buildings 
from  the  effects  of  lightning,  and  also  shows  the  use  of 
pointed  rods  as  tranquil  conductors  of  electricity.  The 
conductor  C  D  is  broken  at  A  and  B  by  two  little  square 
slips  of  wood  having  conducting  wires  passing  through 
them,  and  which  may  be  inserted  in  their  places,  either 
with  the  conducting  wire  broken,  as  at  B  in  the  figure, 
or  with  the  conducting  wire  unbroken,  as  at  A ;  the  ball 
C  may  be  screwed  off  the  wire,  and  then  it  is  terminated 
by  a  point. 

To  use  the  apparatus,  first  let  the  ball  C  be  screwed  on 
the  top  of  the  conducting  wire,  and  let  the  square  slips  be  placed  as  in 
the  figure ;  connect  the  extremity  D  of  the  conducting  wire  with  the 
outer  coating  of  a  charged  jar  ;  place  one  knob  of  the  jointed  discharger 
within  striking  distance  of  the  ball  C,  and  gradually  bring  the  other 
knob  of  the  discharger  within  striking  distance  of  the  knob  of  the  jar ; 
the  disruptive  effect  of  the  charge  will  throw  out  the  slip  B,  while  A 
remains  in  its  place. 

Perform  the  same  experiment  when  the  ball  C  is  taken  off :  the  charge 
will  pass  quietly  through  the  point,  and  both  slips  will  remain  in  their 
place. 

Exp.  2.  The  electric  bomb.  —  A  cavity  is  made  in  a 
block  of  wood  C,  and  closed  by  a  cork  D  ;  two  wires 
A  and  B  pass  into  this  cavity,  having  their  points  about 
a  quarter  of  an  inch  asunder.  Now  connect  the  knob 
B  with  the  exterior  coating  of  a  jar  or  battery  ;  and 
with  the  knob  of  the  discharging  rod  in  contact  with 
the  knob  A,  discharge  the  jar  or  battery,  and  the  cork  will  be  forcibly 
projected  from  the  cavity. 


Fig.  79. 


ELECTRICITY.  257 

Fill  up  the  cavity  with  sand  ;  transmit  a  charge  through  it ;  and  the 
passage  of  the  spark  will  disperse  the  sand  in  all  directions. 

Exp.  3.  Dispersion  of  water.  —  Transmit  a  strong  charge  through  the 
fluid  :  it  will  be  scattered  in  all  directions. 

Exp.  4.    To  perforate  glass.  —  Fill  a  vial  A  (see  Fig. 
80)  with  oil ;  close  it  with  a  cork,  through  which  a  wire 
B  passes,  having  its  lower  end  so  bent  that  its  point  shall 
touch  the  inner  surface  of  the  vial.    Connect  the  extremity 
B  with  the  outside  coating  of  a  charged  jar ;  place  the  knob    ~~Fjg.  80. 
C  of  the  jointed  discharger  opposite  to  the  point,  then  dis- 
charge the  jar,  and  the  spark  in  its  passage  through  the  glass  will  make 
a  hole. 

This  experiment  may  also  be  performed  by  suspending  the  vial  from 
the  prime  conductor  of  a  powerful  machine,  and  taking  the  spark  from 
the  point  by  bringing  a  brass  ball  opposite  to  it. 

Exp.  5.  To  break  wood  and  glass.  —  Transmit  a  strong  charge  through 
a  stick  of  wood,  in  the  direction  of  its  fibres,  about  half  an  inch  thick  : 
the  wood  will  be  split. 

Discharge  a  jar  or  battery  through  a  plate  of  window  glass,  after  the 
manner  described  at  page  2o2,  Exp.  9  :  the  glass  will  be  broken. 

Exp.  6.  To  rupture  substances  which  are  imperfect  conductors  of  elec- 
tricity. —  Place  several  dry  cards  together  between  the  knobs  of  the  uni- 
versal discharger ;  pass  a  strong  charge  through  them,  and  the  spark  will 
pierce  a  hole  through  them.  The  cards  will  have  a  peculiar  sulphurous 
odor,  like  that  which  is  perceived  in  places  after  they  have  been  struck 
by  lightning. 

Thin  pieces  of  wood  may  be  ruptured  in  the  same  manner. 
Place  a  piece  of  dry  writing  paper  on  the  stage  of  the  universal  dis- 
charger, lay  its  knobs  on  the  paper,  at  the  distance  of  an  inch  and  a 
half  from  each  other  ;  then  transmit  the  charge,  and  the  passage  of  the 
spark,  if  sufficiently  strong,  will  tear  the  paper  asunder. 

Lay  a  piece  of  perforated  tin  foil  between  two  panes  of  glass ;  fix  them 
tightly  together,  and  transmit  a  strong  charge  through  the  tin  foil :  the 
panes  of  glass  will  be  split  by  the  discharge. 

Exp.  7.   An  electrical    thermometer,    sometimes    called    a         © 
thermo-electroscope.  —  This   piece  of  apparatus,  represented 
in  Fig.  81,  is  intended  to  show  the  momentary  expansion 
of  the  air  produced  by  the  heat  of  the  spark  in  its  passage      A  Jg 
through  the  air.    A  is  an  air-tight  tube  communicating  with 
a  small  tube  B  which  is  open  at  the  top ;  a  and  b  are  two 
knobs  attached  to  the  extremities  of  wires  passing  out  of  the 
tube  ;  a  colored  liquid  below  the  level  of  the  knob  b  stands 
at  the  same  height  in  the  two  tubes.     When  a  charge  or     Fig.  81, 
22* 


258          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

spark  passes  from  a  to  b,  the  air  in  A  expands  by  the  heat  developed 
by  the  passage  of  the  spark ;  the  liquid  in  A  will  therefore  fall,  while 
that  in  B  will  rise.  The  strength  of  the  electric  charge  is  indicated  by 
the  amount  of  expansion. 


HEATING   EFFECTS    OF    ELECTRIC    DISCHARGES. 

33.  Exp.  1.  Ignition  of  resin  upon  water.  —  Sprinkle  some  powdered 
resin  on  the  surface  of  water  contained  in  a  cup ;  connect  the  outer  coat- 
ing of  a  charged  jar,  by  means  of  a  chain,  with  the  water  in  the  cup ; 
discharge  the  jar,  by  causing  the  spark  to  pass  through  the  resin,  which 
will  instantly  ignite. 

Various  other  substances  may  be  ignited  in  a  similar  manner. 

Exp.  2.  Place  a  skein  of  cotton,  impregnated  with  any  resinous  pow- 
der, on  the  stage  of  the  universal  discharger ;  pass  the  spark  through  the 
cotton,  and  it  will  be  ignited.  This  is  another  way  of  performing  Exp. 
8,  explained  at  page  252. 

Exp.  3.  Explosion  of  gunpowder.  —  The  igniting  power  of  an  electric 
spark  is  increased  by  passing  the  charge  through  a  damp  conductor.  In 
this  way  wre  are  enabled  to  fire  gunpowder,  which  cannot  be  ignited  by 
the  spark  under  ordinary  circumstances  ;  place  some  fine  gunpowder  in 
the  wooden  cup  C,  (Fig.  79  ;)  carry  the  fluid  for  about  six  inches  along 
a  damp  thread  attached  to  that  arm  of  the  discharger  which  is  con- 
nected with  the  outer  coating  of  the  jar :  then  the  passage  of  the  spark 
from  the  end  of  one  wire  to  the  end  of  the  other  will  ignite  the 
powder. 

Here  the  moist  thread,  being  a  somewhat  imperfect  conductor,  retards 
the  passage  of  the  electric  fluid,  and  thereby  causes  the  discharge  to  take 
place  with  less  rapidity  than  it  would  otherwise  do. 

Exp.  4.  A  fine  wire  heated,  fused,  and  burned.  —  Stretch  a  few  inches 
of  very  fine  harpsichord  wire  between  the  ends  of  the  universal  dis- 
charger, (see  Fig.  33  ;)  send  a  good  charge  through  the  -wire,  and  it  will 
be  either  rendered  incandescent,  or  it  will  be  fused.  The  length  of  wire 
which  may  be  fused  depends  upon  the  size  of  the  battery  and  the  inten- 
sity of  the  charge.  A  battery  composed  .of  half  a  dozen  ordinary  jars, 
and  fully  charged  by  a  good  machine,  will  readily  fuse  about  six  inches 
of  fine  harpsichord  wire. 

The  heating  effects  of  electrical  charges  on  different  metals  depend 
on  their  conducting  powers ;  thus  platinum  and  iron,  which  are  bad 
conductors  of  electricity,  become  more  powerfully  heated  by  the  passage 
of  an  electrical  charge  than  gold  and  copper,  which  are  good  con- 
ductors. 


ELECTRICITY. 


259 


The  thermo-electroscope,  represented  by  Fig.  82, 
depends  upon  this  principle.  C  D  A  B  has  the 
form  of  a  differential  thermometer ;  a  platinum  wire 
passes  through  the  ball  C,  and  is  hermetically  sealed 
to  it.  When  an  electric  charge  is  transmitted 
through  the  platinum  wire,  it  becomes  heated,  and 
this  causes  the  air  in  the  ball  C  to  expand,  which  is 
instantly  made  manifest  by  the  rise  of  the  liquid  hi 
the  tube  A  B.  The  graduated  scale  on  AB  gives 
the  relative  heating  powers  of  different  charges. 
This  instrument  is  best  adapted  to  the  measurement 
of  the  heating  power  of  voltaic  electricity. 

Exp.  5.   Ignition  and  fusion  of  gold  leaf.  — Plac 
a  strip  of  gold  leaf  between  two  pieces  of  dry  paper ; 
lay  them  on  the  table  of  the  universal  discharger; 
pass  a  good  charge  through  the  gold  leaf,  and  it  will 
be  burnt.    Both  pieces  of  paper  will  be  covered  with 
a  purple  strip  of  oxide  of  gold ;  the  strip  has  a  grayish  tinge  when  the 
gold  leaf  contains  a  portion  of  silver. 

Exp.  6.  Place  a  small  bit  of  gold  leaf  between  two  pieces  of  window 
glass  ;  proceed  as  in  the  last  experiment,  and  the  gold  will  be  fused  into 
the  glass. 

Exp.  7.  Ignition  of  gilt  thread.  —  Stretch  a  gilt  thread  of  silk  be- 
tween the  extremities  of  the  universal  discharger ;  send  a  charge  through 
the  thread,  and  the  electric  fluid,  in  its  passage,  will  bum  the  gilding, 
and  the  silk  will  remain  uninjured. 


Fig.  82. 


PHYSIOLOGICAL    EFFECTS    OF   ELECTRIC    DISCHARGES. 

34.  The  sensation  of  a  spider's  web  being  drawn  over  the 
face,  and  the  peculiar  phosphoric  odor  attending  the  transmis- 
sion of  electricity,  are  amongst  the  most  ordinary  physiologi- 
cal effects  of  electricity.  When  a  strong  electrical  charge 
passes  through  the  body,  it  is  accompanied  by  a  shuddering 
sensation  and  a  sudden  contraction  of  the  muscles,  which  is 
called  the  electric  shock.  (See  Exp.  4,  page  231.)  The  dis- 
charge from  a  single  jar  is  sufficient  to  destroy  the  life  of 
small  animals ;  and  the  discharge  of  a  powerful  battery 
through  the  head  of  a  large  animal  is  enough  to  kill  it. 


2GO  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Exp.  1.  In  taking  a  shock  from  a  jar,  interpose  in  some  part  of  the 
circuit  a  damp  rope  :  then,  instead  of  the  usual  shock,  there  will  be 
merely  a  tingling  sensation  produced  at  the  tips  of  the  fingers. 

Exp.  2.  Place  the  head  of  a  live  mouse  between  the  wires  of  the  uni- 
versal discharger  ;  send  a  strong  shock  through  it,  and  the  mouse  will  be 
instantly  killed. 

MAGNETIC    EFFECTS    OF   ELECTRIC    DISCHARGES. 

35.  Exp.  1.  Place  a  small  sewing 
needle  in  a  helix  or  spiral  formed  of 
copper  wire,  a  b,  (Fig.  83,)  covered 
over  with  silk ;  place  the  ends  of  the  Fig.  83. 

helix  in  contact  with  the  arms  of  the 

universal  discharger ;  transmit  a  strong  charge  through  the  wire,  and  the 
needle  will  be  rendered  magnetic.  The  end  of  the  needle  which  lies  to 
the  right  of  the  electric  current  will  be  a  north  pole,  and  the  opposite 
end  a  south  pole. 

Exp.  2.  Reverse  the  direction  of  the  needle  of  the  last  experiment ; 
transmit  two  or  more  charges  of  electricity  through  the  helix,  and  the 
poles  of  the  needle  will  be  reversed. 

The  magnetic  effects  of  common  electricity  are  very  feeble 
as  compared  with  those  of  voltaic  electricity.  The  explana- 
tion of  these  phenomena  will  be  given  in  connection  with  the 
subject  of  galvanism. 


CHEMICAL    EFFECTS    OF    ELECTRIC    DISCHARGES. 

36.  The  chemical  effects  of  the  ordinary  electric  currents, 
like  the  magnetic  effects,  are  comparatively  feeble.  The  fol- 
lowing experiments,  however,  show  that  ordinary  electricity 
really  possesses  a  decomposing  influence. 

Exp.  I.  Place  two  pieces  of 
tin  foil  1 1  on  a  dry  pane  of 
glass  G  G ;  on  these  pieces  of 
tin  foil  lay  platinum  wires,  bent 
in  the  manner  shown  in  Fig. 
84,  so  that  there  shall  be  a 
small  space  between  the  two  points  at  k,  where  they  touch  the  glass,  and 


ELECTRICITY.  261 

where  the  body  which  is  to  be  decomposed  is  placed.  Lay  the  glass  G  G 
on  the  table  of  the  universal  discharger  ;  place  its  two  knobs  on  the  tin 
foils,  and  connect  one  of  them  by  a  chain  and  a  moist  thread  with  the 
prime  conductor  of  the  machine,  and  the  other  with  the  insulated  cush- 
ion. Place  a  drop  of  a  solution  of  iodine  of  potassium  at  k,  between  the 
platinum  points ;  turn  the  machine,  and  after  a  short  time  the  iodine  will 
be  deposited  at  the  positive  wire,  and  the  metallic  potassium  at  the  neg- 
ative wire.  Perform  the  same  experiment  with  a  drop  of  a  solution  of 
sulphate  of  copper,  and  so  on. 

These  experiments  may  be  performed  with  more  delicacy  by  using 
blotting  paper  saturated  with  the  solutions  ;  thus  paper  dipped  in  a  solu- 
tion of  iodine  in  alcohol  will  readily  give  a  blue  tinge  of  iodine  on  the 
paper  in  contact  with  the  positive  wire. 

The  decomposition  of  water  by  common  electricity  was  first  shown  by 
Wollaston. 

Sparks  discharged  for  a  length  of  time  through  the  air  of  a  closed 
receiver  cause  the  two  gases  in  the  air  'to  combine  and  form  nitric 
acid  ;  in  this  way,  no  doubt,  nitric  acid  is  formed  in  the  atmosphere  by 
lightning. 

Exp.  2.  Place  a  fine  metal  point  in  connection  with  the  prime  con- 
ductor of  the  machine ;  work  the  machine  for  some  time,  and  then  bring 
the  metal  point  in  contact  with  the  tongue  :  a  faint  acid  taste  is  felt ; 
whereas  the  negative  electricity  will  produce  an  alkaline  taste. 

DISTRIBUTION    OF    ELECTRICITY. 

37.  The  electric  fluid  arranges  itself  upon  the  surfaces  of 
conductors. 

Exp.  1.  A  is  an  electrified  metal  ball,  suspended  by  a  silk  thread  ;  B 
and  C  are  two  hollow  metal  hemispheres,  which  exactly  envelop  the 


sphere  ;  when  they  are  removed  from  the  sphere,  then  not  the  slightest 
trace  of  electricity  remains  upon  it,  while  the  outer  surfaces  of  the  hemi- 


262 


NATURAL.  AND    EXPERIMENTAL    PHILOSOPHY. 


spheres  contain  all  the  electricity  which  was  at  first  in  A.     This  may  be 
proved  by  means  of  the  electroscope. 


The  Proof  Plane. 

To  show  in  a  more  complete  manner  the  superficial  distribu- 
tion of  electricity,  a  small  piece  of  apparatus,  called  a  proof 
2)lane,  is  usually  employed.  This  apparatus  is  represented  in 
Fig.  86,  where  C  is  a  small  disk  of  gilt  paper,  fixed  at  the  end 
of  a  stick  of  gum  lac  A  B.  In  using  this  instrument,  a  point 
of  the  electrified  surface  is  touched  by  the  proof  plane,  which 
being  carried  to  the  torsion  electrometer,  the  intensity  of  the 
electricity  at  the  point  touched  by  the  proof  plane  is  indicated 
by  the  deflection  of  the  needle.  _,. 

Exp.  2.  A  is  a  conical  muslin  bag,  fixed  to  an  insulated  metal 
ring,  forming  something  like  a  butterfly 
net ;  B  and  C  are  silk  threads  attached 
to  the  apex  of  the  cone,  one  on  the  out- 
side and  the  other  on  the  inside,  by 
which  the  cone  may  be  turned  outside  in. 
Let  the  cone  be  charged  with  electricity 
by  means  of  a  carrier  ball ;  test  the  elec- 
tricity of  the  inside  and  outside  surfaces 
by  means  of  the  proof  plane ;  then  it  will 
be  found  that,  while  the  outside  surface 
is  charged  with  electricity,  the  inside  sur- 
face is  entirely  free  from  it.  Turn  the 
cone  outside  in,  and  test  the  surfaces  as 
before ;  the  surface  which  is  now  outside  will  contain  all  the  electricity, 
and  that  which  is  now  inside  will  be  entirely  free  from  it. 

These  experiments  clearly  show  that  the  electricity  distributes  itself 
upon  the  exterior  surface  of  a  conducting  body,  but  not  on  the  interior 
surface. 

The  following  experiment,  first  given  by  Faraday,  establishes  the  same 
principle,  as  well  as  an  important  law  relative  to  the  induction  of  elec- 
tricity :  *  — 


Fig.  87. 


*  By  applying  his  theoretical  ideas  to  other  and  different  phenomena  of 
statical  electricity,  Faraday  is  led  to  admit  that  the  tendency  of  electricity  to 
distribute  itself  on  the  surface  of  conducting  bodies  is  more  apparent  than 
real,  and  that  the  experiments  which  prove  that  there  is  not,  in  fact,  any  free 
electricity  except  at  their  surface,  are  easily  explained  in  another  manner. 
No  electric  charge,  according  to  this  theory,  can  be  manifested  in  the  inte- 


ELECTRICITY. 


263 


0 


An  insulated  electrified  ball  A  is  sus- 
tained in  the  interior  of  a  series  of  jars, 
placed  the  one  within  the  other,  and  sep- 
arated from  each  other  by  plates  of  gum 
lac,  as  shown  in  Fig.  88  ;  the  outer  jar 
B  communicates  with  a  gold  leaf  electro- 
scope C,  the  leaves  of  which  L  diverge 
the  moment  the  electrified  ball  A  is  in- 
troduced. Here  induction  takes  place 
from  jar  to  jar,  until  at  last  the  outer  sur- 
face of  the  jar  B  becomes  electrified. 

Upon  testing  the  electricity  on  the  sur 
face  of  the  jars  by  means  of  the  proof 
plane,  it  will  be  found,  while  the  outer 
surfaces  of  the  jars  all  contain  electricity 
the  inner  surfaces  are  entirely  free  from  it. 

While  the  gold  leaves  L  are  divergent, 
let  the  electrified  ball  A  touch  the  side  of 
the  inner  jar,  and  it  of  course  transmits  Fig.  88. 

its  electricity  to  the  jar,  and  the  gold 

leaves  neither  diverge  more  nor  less  than  before.  This  experiment  proves 
that  the  electricity  possessed  by  the  ball  is  exactly  equal  in  quantity  and 
in  power  to  that  which  it  develops  by  induction. 


rior  of  a  body,  on  account  of  the  opposite  directions  of  the  electricities  in 
each  of  the  interior  particles;  whence  the  resulting  effect  is  null ;  whilst  the 
induction  exercised  by  exterior  bodies  renders  the  electricity  sensible  on  the 
surface.  From  this  manner  of  regarding  it,  electricity  must  show  itself  only 
on  the  surface  of  a  conducting  envelop,  whatever  be  its  conductibility  or  the 
insulating  property  of  the  substance  placed  within.  Faraday,  in  fact,  de- 
monstrated this  by  strongly  electrizing  oil  of  turpentine  placed  in  a  metal 
vessel.  There  was  no  apparent  electricity,  except  on  the  exterior  surface  of 
the  vessel.  He  also  constructed  a  cubical  chamber,  twelve  feet  square,  the 
wooden  sides  of  which  were  covered  outside  with  tin  foil ;  he  insulated  it ; 
then,  after  having  introduced  into  it  electroscopes  and  other  objects,  he  elec- 
trized the  interior  air  with  a  strong  machine.  No  trace  of  electricity  was 
manifested  within  ;  whilst  considerable  sparks  and  luminous  brushes  darted 
off  in  all  directions  from  the  exterior  surface.  "While  these  experiments 
complete  those  of  Coulomb,  in  which  he  operated  only  upon  conducting  bod- 
ies, they  render  the  explanation  that  was  given  rather  improbable,  since  it 
was  based  upon  the  free  propagation  of  electricity  in  the  conducting  mass  ; 
whence  it  followed  that  this  electricity  distributed  itself  entirely  on  the  sur- 
face. When  once  the  phenomenon  has  occurred  in  the  same  manner  with 
insulating  bodies  placed  interiorly,  this  explanation  is  not  tenable. 
With  regard  to  the  influence  of  form  upon  the  quantity  of  electricity 


264          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

The  intensity  of  the  electricity  upon  a  conducting  body 
depends  upon  the  extent  of  that  surface. 

Exp.  3.  A  B  is  an  insulated  me- 
tallic roller,  which  may  be  turned 
by  the  insulated  handle  H  ;  D  is  a 
pith  ball  electroscope ;  C  is  a  me- 
tallic ribbon  coiled  upon  the  roller. 
Let  the  roller  be  charged  with  elec- 
tricity, then  the  balls  D  will  diverge 
from  each  other,  indicating  the  in- 
tensity of  the  charge ;  let  the  me- 
tallic ribbon  be  unrolled,  drawing  Fig.  89. 
it  by  means  of  a  silk  thread  at- 
tached to  the  extremity  C ;  then  the  balls  D  will  approach  each  other, 

accumulated  at  the  surface  of  bodies  not  spherical,  it  would  always  depend, 
according  to  Faraday's  theory,  upon  some  points  of  the  surface  being  ex- 
posed to  a  greater  amount  of  inductive  forces  than  others.  Thus  the  extrem- 
ities of  a  cylinder,  or  of  an  elongated  ellipsoid,  would  be  more  strongly  elec- 
trized than  the  rest  of  the  surface,  because  there  go  from  them  a  greater 
number  of  filaments  of  polarized  particles,  establishing  with  surrounding 
conductors  the  communication  necessary  for  induction.  A  point  is  far  su- 
perior in  this  respect ;  for  it  is  the  centre  whence  emanate  in  all  directions 
the  lines  of  inductive  force,  which,  for  example,  when  a  ball  is  in  question, 
are  found  distributed  over  a  greater  extent,  and  do  not  set  out  from  a  single 
point  only,  but  equally  from  all  points  of  its  surface. 

In  the  theory  that  we  have  been  explaining,  the  mutual  repulsion  of  bodies 
charged  with  the  same  electricity  is  only  apparent ;  it  is  called  into  existence 
because  there  is  no  electricity  on  the  nearer  surfaces,  and  because  each  of 
the  bodies  is  attracted  in  opposite  directions  by  the  surrounding  bodies,  upon 
which  induction  determines  an  electrical  state  dissimilar  to  their  own.  We 
may  even  prove,  by  means  of  the  proof  plane,  that  the  two  gold  leaves  of  an 
electroscope,  when  they  are  diverging,  have  no  electricity  on  their  interior  sur- 
face, whilst  they  are  strongly  electrized  exteriorly,  however  thin  they  may  be 
in  other  respects.  Repulsion  is  also  explained  by  attributing  it  to  the  attrac- 
tion exercised  upon  each  of  the 'gold  leaves  by  the  contrary  electricity, 
developed  by  induction,  in  the  strata  of  air  in  contact  with  their  exterior 
surface.  This  mode  of  action*  of  the  air  is  much  more  natural  and  more 
probable  than  that  in  which  it  is  regarded  as  determining  repulsion  by  the 
greater  pressure  from  within  outwards,  than  inwards  from  without,  which  it 
exercises  upon  electrized  bodies.  However,  the  experiments  which  show  that 
repulsion  takes  place  in  vacua  as  well  as  air,  would  seem  to  be  equally  con- 
trary to  these  two  explanations,  except  that,  in  the  former,  we  admit  the  effect 
by  induction  of  the  ambient  bodies,  even  when  they  are  placed  at  a  great 
distance. 


ELECTRICITY.  265 

owing  to  the  electricity  having  become  spread  over  a  greater  extent  of 
surface ;  now  let  the  ribbon  be  rolled  up,  by  the  insulated  handle  H,  and 
the  pith  balls  will  again  diverge  from  each  other. 

Exp.  4.  To  show  that  electricity  accumulates  itself  towards  the  extrem- 
ities of  an  insulated  conductor.  —  Touch  the  different  parts  of  the  elec- 
trified conductor  with  the  proof  plane,  and  test  the  intensity  of  the 
electricity  in  each  case  by  means  of  the  torsion  electrometer,  and  it  will 
be  found  that  those  parts  of  the  conductor  which  are  farthest  from  the 
middle  have  the  greatest  intensity.  Hence  the  tendency  of  the  electric 
fluid  to  escape  from  pointed  extremities.  These  effects  apparently  arise 
from  the  mutual  repulsion  of  the  particles  of  the  fluid. 

ATMOSPHERIC   ELECTRICITY. 

THE   IDENTITY   OP   ELECTRICITY   AND   LIGHTNING. 

38.  The  ho,nor  of  this  discovery  belongs  to  Franklin.  In 
a  letter  to  a  friend  he  gives  the  following  account  of  the  ori- 
gin of  the  conception  which  conducted  him  to  the  great  dis- 
covery :  "  Your  question,  how  I  came  first  to  think  of  pro- 
posing the  experiment  of  drawing  down  the  lightning  in 
order  to  ascertain  its  sameness  with  the  electric  fluid,  I  can- 
not better  answer  than  by  giving  you  an  extract  from  the 
minutes  I  used  to  keep  of  the  experiments  I  made,  with 
memorandums  of  such  as  I  purposed  to  make,  the  reasons 
for  making  them,  and  the  observations  that  arose  upon  them, 
from  which  minutes  my  letters  were  afterwards  drawn.  By 
this  extract  you  will  see  that  the  thought  was  not  so  much 
an  out  of  the  way  one,  but  that  it  might  have  occurred  to  an 
electrician.  *  Nov.  1749.  Electric  fluid  agrees  with  lightning 
in  these  particulars :  1.  Giving  light;  2.  Color  of  the  light; 
3.  Crooked  direction  ;  4.  Swift  motion ;  5.  Being  conducted 
by  metals  ;  6.  Crack  or  noise  in  exploding ;  7.  Subsisting  in 
water  or  ice ;  8.  Rending  bodies  it  passes  through ;  9.  De- 
stroying animals  ;  10.  Melting  metals;  11.  Firing  inflammable 
substances ;  12.  Sulphureous  smell.  The  electric  fluid  is  at- 
tracted by  points.  We  do  not  know  whether  this  property  is 
in  lightning,  but  since  they  agree  in  all  the  particulars  in 
23 


266  NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

which  we  can  already  compare  them,  is  it  not  probable  they 
agree  likewise  in  this  ?  Let  the  experiment  be  made.'  " 

This  letter  will  always  be  read  with  interest,  affording,  as  it 
does,  one  of  the  most  admirable  examples  of  inductive  rea- 
soning. 

Franklin  made  the  experiment  in  the  following  manner. 
He  made  a  kite  with  points  fixed  to  it,  with  the  view  of 
drawing  electricity  from  the  clouds.  In  order  to  insulate  the 
electricity  that  might  pass  down  the  hempen  cord,  which  is  a 
partial  conductor  of  electricity,  he  attached  a  silk  cord  to  its 
extremity,  where  he  placed  a  key,  from  which  he  expected  to 
obtain  sparks  of  electricity.  Afraid  of  being  laughed  at, 
should  his  experiment  fail,  he  took  his  little  boy  with  him,  to 
make  it  appear  as  if  he  were  going  to  assist  the  boy  in  flying 
his  kite.  Franklin  and  his  little  boy  having  raised  their  elec- 
trical kite  in  the  air,  they  waited  a  long  time  before  any  indi- 
cations of  electricity  could  be  seen.  At  length  a  thunder 
cloud  passed  over  the  kite ;  the  electric  fluid  passed  from  the 
cloud  to  the  points  fixed  on  the  kite,  and  descended  the 
hempen  cord,  the  fibres  of  which  stoQd  erect  by  electrical 
repulsion ;  Franklin  then  applied  his  knuckle  to  the  key,  and 
received  the  electric  spark. 

What  must  have  been  the  ecstasies  of  his  soul  at  that  mo- 
ment !  He  had  made  one  of  the  most  brilliant  discoveries  in 
the  whole  range  of  physical  science !  he  had  discovered  the 
identity  of  lightning  and  electricity ! 

He  afterwards  charged  Leyden  jars  with  lightning,  and 
made  other  experiments,  similar  to  those  usually  performed 
with  electrical  machines.  He  also  introduced  lightning  con- 
ductors, or  pointed  rods,  for  the  protection  of  buildings  from 
the  effects  of  lightning.  (See  Exp.  1,  page  256.) 

The  picture  of  Franklin  and  his  little  boy  flying  the  kite 
which  first  drew  lightning  from  the  clouds,  will  be  regarded 
with  interest  to  the  latest  ages  of  the  world. 

About  the  same  time,  acting  under  Franklin's  suggestion, 
Dalibard  erected  an  insulated  pointed  rod,  40  feet  high,  and 
thereby  succeeded  in  obtaining  sparks  from  the  clouds. 


ELECTRICITY. 


267 


Fig.  00. 
ELECTRICITY   IN    THE    AIR. 

39.  Electricity  is  always  found  in  the  air,  but  it  varies 
both  in  kind  and  in  quantity.  It  is  generally  positive  when 
the  air  is  clear  and  serene,  and  negative  when  it  is  humid 
and  cloudy.  The  intensity  of  electrical  phenomena  is  usually 
greatest  in  the  higher  strata  of  the  atmosphere :  it  is  also 
stronger  in  winter,  especially  during  frosty  weather,  than  it  is 
in  summer,  and  when  the  air  is  calm  than  when  it  is  bois- 
terous. When  the  wind  blows  from  the  north,  the  drops  of 
rain  are  generally  positive,  and  when  it  blows  from  the  south, 
they  are  generally  negative.  The  earth  is  always  in  a  con- 
trary state  of  electricity  to  that  of  the  higher  strata  of  the 
atmosphere  ;  and  hence  the  atmosphere,  at  the  height  of  a  few 
feet  above  the  surface,  is  always  in  a  neutral  state.  The  aerial 


268  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

electricity  attains  a  maximum  and  minimum  condition  twice 
every  day ;  its  intensity  is  least  during  the  night ;  it  increases 
after  sunrise,  or  during  the  fall  of  dew,  and  attains  its  maxi- 
mum condition  a  few  hours  after  sunrise :  from  that  time  it 
gradually  decreases  until  a  few  hours  before  sunset,  when  it 
reaches  its  second  minimum  condition ;  after  sunset  it  rises 
rapidly,  especially  during  the  fall  of  dew,  and  attains  its  sec- 
ond maximum  condition  a  few  hours  after  sunset. 


ELECTROMETEORS. 

40.  The  most  common  electrometeors  are  thunder  storms, 
sheet  lightning,  the  aurora  borealis,  waterspouts,  whirlwinds, 
and  the  luminous  appearance  of  pointed  conductors.     The 
commonest  and  grandest  of  these   electrical  phenomena   are 
thunder  and  lightning. 

THE   AURORA    BOREALIS,    OR   NORTHERN    LIGHTS. 

41.  In  the  higher  regions  of  the  atmosphere,  where  the 
air  is  very  much  attenuated,  the  flashes  of  electric  light  give 


rise  to  the  well-known  phenomenon  of  the  aurora  borealis,  or 
northern  lights.  (See  Exp.  2,  page  238.)  This  meteor  is  seen 
most  brilliantly  towards  the  arctic  regions.  Fig.  91  repre- 


ELECTRICITY. 


269 


scnts  the  appearance  which  it  presents  at  its  commencement, 
where  streams  of  electric  light  appear  to  move  from  the 
northern  parts  of  the  horizon  towards  the  magnetic  zenith. 
Sometimes,  even  with  us,  it  assumes  the  form  of  a  magnifi- 
cent luminous  bow,  spanning  the  horizon  for  thirty  or  forty- 
degrees. 

Figs.  92  and  93  represent  some  of  the  appearances  of  the 


Fig.  92. 

aurora  borealis  at  the  north  arctic  zone,  as  given  by  M.  Lottin, 
an  officer  of  the  French  navy. 


Fig.  94  represents  a  remarkable  appearance  of  the  aurora 
borealis,  which  was  seen  over  every  part  of  Europe.     This 
was  observed  and  described  by  Mairan  in  the  year  172G. 
23* 


270  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  94. 


Fig.  95. 


^r^r-  W**r , 


Fig.  96. 


ELECTRICITY.  271 

WATERSPOUTS. 

42.   At  the  commencement  of  this  wonderful  and  terrific 
phenomenon,  the  watery  vapor  in  the  clouds  appears  to  de- 


Fiy.  97. 

scend  in  the  form  of  a  cone,  while  the  ocean  beneath  becomes 
agitated,  as  shown  in  Fig.  95  ;  the  apex  of  the  cone  continues 
to  descend,  and,  after  a  little  time,  a  cloud  of  watery  vapor 
rises  from  the  ocean  towards  it,  as  shown  in  Fig.  9G.  This 
goes  on  until  the  two  streams  of  watery  vapor  join  each  other 
and  form  a  complete  waterspout,  or,  it  may  be,  form  two  or 
more  waterspouts,  as  shown  in  Fig.  97. 

These  remarkable  phenomena  appear  to  be  due  to  the  dif- 
ferent electrical  conditions  of  the  cloud  above  and  the  ocean 
beneath. 

DIFFERENT  MODES  OF  GENERATING  ELECTRICITY. 

43.  Besides  friction,  there  are  various  modes  of  generating 
electricity.  The  following  are  amongst  the  most  remark- 
able :  — 


272 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


ELECTRICITY    GENERATED    BY    THE    FRICTION    OF    HIGH 
PRESSURE    STEAM. 

The  friction  of  high  pressure  steam  on  the  metallic  pipes, 
&c.,  through  which  it  is  made  to  pass,  has  recently  been  found 
to  develop  large  quantities  of  electricity. 

A  very  powerful  electrical  machine  has  been  constructed  on  this  prin- 
ciple by  Mr.  Armstrong  of  Newcastle,  and  called  by  him  the  Hydro- 
electric machine. 


HYDRO-ELECTRIC    MACHINE. 


44.   This  machine  is  represented  in  Figs.  98  and  99.     A  is  a  strong 
steam  boiler,  cased  in  wood  to  reduce  the  radiation  of  heat,  standing  on 


Fig.  98. 

six  glass  pillars  O ;  B  the  furnace,  and  C  the  ash  pit,  formed  in  the 
under  part  of  the  boiler ;  P  Q  the  chimney ;  D  is  a  water  gauge,  and  E 


ELECTRICITY. 


273 


the  feed  valve ;  F  F  two  tubes  leading  from  the  valves  1 1  to  the  large 
tubes  G  G  ;  H  II  are  a  series  of  bent  iron  tubes,  proceeding  from  the 
pipes  G  G,  and  terminating  in  jets  J,  which  may  be  opened  or  closed, 
by  means  of  levers  placed  at  K  K ;  M  is  the  safety  valve. 

Fig.  99  represents  a  zinc  case,  provided  with  four  rows  of  brass  points, 
which  are  placed  in  front  of  the  rows  of  the  jets  J,  (Fig.  98,)  in  order  to 


Fig.  99 

attract  the  electricity  from  the  steam  vapor  projected  upon  them  :  when 
long  sparks  are  required,  this  case,  with  its  points,  is  placed  at  the  dis- 
tance of  about  one  foot  from  the  jets ;  and,  on  the  contrary,  when  a 
large  quantity  of  electricity  is  required,  the  case  is  brought  within  a  few 
inches  of 'the  jets.  With  a  view  of  augmenting  the  development  of  the 
electricity,  the  inner  surfaces  of  the  jets  are  lined  with  w*ood,  forming  a 
bent  channel  for  the  passage  of  the  steam. 

In  this  machine,  we  may  regard  the  particles  of  water  as  serving  the 
purpose  of  the  glass  plate  of  a  common  electrical  machine  ;  the  wooden 
lining  of  jets  as  the  rubber  ;  and  the  steam  as  the  rubbing  power. 

The  electricity  generated  by  this  engine  is  more  remarkable  for  its 
enormous  quantity  than  for  its  high  intensity.  The  engine  erected  by 
Mr.  Armstrong,  at  the  Polytechnic  Institution,  gave  sparks  from  twelve 
to  fourteen  inches  in  length,  and  charged  a  battery,  containing  80  feet 
of  coated  glass,  in  ten  seconds.  The  dense  sparks,  which  pass  from  the 
boiler  to  any  large  ball  conductor,  follow  each  other  in  such  a  rapid 
succession,  as  to  give  to  them  somewhat  of  the  character  of  a  galvanic 
flame. 


ELECTRICITY  DEVELOPED  BY  CONTACT. 

45.  When  two  different  metals  are  brought  into  contact,  electricity  is 
developed  ;  the  positive  fluid  being  attached  to  the  one  metal,  and  the  neg- 
ative fluid  to  the  other.  C  and  Z  are  two  plates  of  copper  and  zinc,  hav- 
ing the  insulating  handles  A  and  B.  Let  them  be  brought  in  contact,  and 


274 


NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 


then  separated,  taking  care  to  hold  them 
by  the  insulating  handles,  and  to  move 
them  towards  and  from  each  other,  so 
that  no  friction  shall  take  place  in  form-  • 
ing  or  breaking  the  contact ;  then  the 
zinc  plate  will  be  charged  with  positive 
electricity,  and  the  copper  plate  with 
negative  electricity ;  which  may  be 
proved  by  bringing  the  plates  in  con- 
tact with  the  connecting  plate  of  the 
condensing  electroscope.  (See  Fig.  G4.) 
This  constitutes  the  fundamental  ex- 
periment of  voltaic  electricity. 


Fig.  101. 


Deluc's  Dry  Piles. 

On  this  principle  Deluc  constructed  his  electric  pile,  which  consisted 
of  a  series  of  disks  of  copper  and  zinc  paper,  laid  the  one  upon  the  other, 
with  their  paper  sides  together.  A  pile  containing  about  1000  pairs  of 
these  disks  exhibits  a  decided  evidence  of  electrical  attraction  and  re- 
pulsion when  a  connection  is  formed  between  the  extreme  plates.  What 
is  remarkable  in  these  dry  piles  is,  that  they  will  remain  with  undimin- 
ished  action  for  years,  without  being  at  all  interfered  with. 


ZambonVs  Electrical  Perpetual  Motion. 

This  beautiful  piece  of  apparatus  is  formed  by  placing  two  of  Deluc's 
piles,  (Fig.  102,)  each  containing  about  1000  pairs  of  plates,  within 
about  two  inches  of  each  other,  so  that  their  unlike  poles  may  be  brought 
near  each  other  at  the  top  and  bottom.  The  upper 
extremities  of  the  piles  terminate  in  two  metal  knobs, 
C  and  D,  and  the  lower  extremities  are  connected 
by  a  strip  of  copper,  so  that  while  one  knob  C  is  pos- 
itive, the  other  knob  D  is  negative.  P  B  is  a  light 
pendulum  rod  of  gum  lac,  turning  on  a  centre  at  A, 
and  its  upper  knob  B  playing  between  the  electrified 
knobs  C  and  D  ;  the  knob  B  of  the  pendulum  is  al- 
ternately attracted  and  repelled  by  the  electrified 
knobs  C  and  D.  This  motion  will  often  continue  for 
years  without  intermission. 


ELECTRICITY. 


275 


Bohnenberg's  Electroscope. 

One  of  the  most  useful  applications  of  the  dry  pile  is  exhibited  in  the 
construction  of  an  electroscope,  represented  in  Pig. 
103,  which  is  not  only  the  most  sensitive  of  all  oth- 
ers, but  has  the  additional  property  of  at  once  indi- 
cating the  peculiar  kind  of  electricity  of  the  body 
applied  to  it. 

This  instrument  consists  of  two  dry  piles  C  and 
D,  placed  as  in  Zamboni's  perpetual  motion ;  be- 
tween the  knobs  C  and  D,  a  single  gold  leaf  G  is 
suspended  in  the  same  manner  as  in  the  ordinary 
gold  leaf  electroscope.  The  moment  the  gold  leaf 
G  is  electrified  by  the  approach  of  any  electrified 
body  towards  A,  it  is  carried  either  towards  one 
knob  or  the  other,  according  to  the  nature  of  the 
electricity  with  which  the  body  is  charged ;  that  is 
to  say,  if  the  electroscope  be  charged  with  positive 
electricity,  then  the  gold  leaf  G  will  be  attracted 
^towards  the  negative  knob  of  the  pile,  and  so  on. 


\  \  v',' 

H 

!  ! 

= 

i  ! 

I  I 

) 

- 
:        ' 
= 
- 

=  § 

;""--. 

1  1 

J 

JV^.  103. 


MAGNETISM. 


THE  MAGNETIC  POWER. 

1.  SUBSTANCES  endowed  with  magnetism  attract  pieces  of 
iron,  and  the  substances  possessing  this  property  are  called 
magnets.  Magnetic  substances  possess  various  other  remark- 
able properties,  which  shall  hereafter  be  described.  There 
are  two  kinds  of  magnets  —  natural  magnets  and  artificial 
magnets. 

Natural  Magnets,  or  loadstones,  are  iron  ores,  found  at  al- 
most every  place  on  the  earth.  The  ancient  Greeks  were 
acquainted  with  the  attractive  property  of  the  natural  magnet, 
or  loadstone  ;  they  gave  the  name  of  magnet  to  this  mineral, 
probably  because  it  was  found  most  abundant  in  the  vicinity 
of  Magnesia,  a  city  of  Lydia,  in  Asia  Minor. 

Artificial  Magnets  are  generally  made  of  steel  bars  ;  and 
the  way  in  which  the  magnetic  property  is  imparted  to  them 
will  shortly  be  described.  Artificial  magnets  are  named 
according  to  their  shape  ;  thus,  we  have  the  bar  magnet,  rep- 
resented in  Fig.  1,  and  the  horseshoe  magnet,  represented  in 


Fig.  1 

Fig.  2.  When  several  bar  magnets  or  horseshoe  magnets 
are  combined,  the  whole  is  called  a  magnetic  battery,  or  a 
compound  magnet. 

The  magnetic  power  of  a  magnetized  bar  chiefly  resides  in 
its  extremities,  which  are  called  the  magnetic  poles ;  one 
being  called  the  north  pole  of  the  magnet,  and  the  other  the 

(276) 


MAGNETISM.  277 

south  pole.  In  order  to  distinguish  these  poles  from  each 
other,  a  mark  is  usually  drawn  across  the  extremity  corre- 
sponding to  the  north  pole  of  the  magnet. 

One  of  the  most  remarkable  properties  of  the  magnet  is, 
that  it  communicates  its  properties  to  a  steel  bar  or  needle 
that  is  rubbed  for  a  few  times,  in  the  same  direction,  across 
one  of  its  poles. 

MAGNETIC   ATTRACTION. 

2.  Exp.  1.  Sprinkle  some  iron  filings  on  a  magnetic  steel  bar ;  the 
iron  filings  will  be  attracted  to-  the  extremities  or  poles  of  the  magnet, 
whilst  the  other  portions  will  be  left  nearly  bare,  as  shown  in  Fig.  3. 


Fig.  3. 

"When  the  steel  bar  exceeds  eight  or  ten  inches  in  length,  we  sometimes 
find  two  other  poles  besides  those  that  are  at  the  ends,  as  shown  in 
Fig.  4. 


Exp.  2.  Attract  a  series  of  pieces  of  iron  wire  a  b  c  to  the  extremity 
N  of  the  magnetic  bar  N  S,  as  shown  in  Fig.  5.  Here  the 
wires,  while  they  are  in  connection  with  the  magnet  N  S,  be- 
come a  series  of  little  magnets,  whose  lower  extremities  are  all 
north  poles ;  that  is,  of  the  same  name  as  the  pole  of  the  mag- 
net to  which  they  are  attached. 

Exp.  3.  To  magnetize  a,  penknife.  — Rub  the  knife,  for  sev- 
eral times,  in  the  same  direction,  that  is,  from  haft  to  point,  across 
one  of  the  extremities,  or  poles,  of  a  magnet ;  apply  the  point 
of  the  knife  to  some  iron  filings,  or  small  pieces  of  iron :  they 
will  be  attracted  to  the  point  of  the  knife.  F.  . 

Tfie  attraction  between  a  magnet  and  iron  is  reciprocal.  — 
Whilst  the  magnet  attracts  iron,  the  iron  also  attracts  the 
magnet. 

24 


278 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Exp.  1.  Suspend  a  piece  of  iron  wire  by  a  thread,  so  that  the  wire 
may  hang  horizontally.  Bring  the  one  extremity  of  a  magnet  near  to 
one  end  of  the  wire ;  the  wire  will  be  attracted  by  the  magnet. 

Exp.  2.  Suspend  a  magnetized  needle  in  the  same  manner  ;  bring  the 
extremity  of  the  iron  wire  near  to  either  pole  of  the  magnet ;  the  mag- 
net will  be  attracted  by  the  iron  wire. 

Magnetic  Attraction  transmitted  through  various  Bodies. 

Exp.  1.  Interpose  a  thin  screen  of  wood,  or  glass,  or  copper,  or  any 
substance  excepting  steel  and  iron,  between  the  magnet  and  the  iron 
wire  of  the  foregoing  experiments ;  the  attraction  will  take  place  just  as 
if  there  were  no  substance  interposed. 

Exp.  2. ,  Strew  some  iron  filings  on  a  sheet  of  white  paper  ;  place  the 
pole  of  a  magnet  beneath  them;  the  filings  will  appear  to  move  in 
whatever  direction  the  magnet  is  moved. 

Exp.  3.  Interpose  an  iron  plate  between  a  magnet  and  an  iron  wire 
suspended  by  a  thread ;  the  magnet  will  have  little  or  no  effect  upon  the 


Distribution  of  Magnetism  in  a  magnetized  Bar. 

The  inequality  of  this  distribution  may  be  readily  proved  by  the  fol- 
lowing experiments. 


Fig.  6. 


MAGNETISM.  279 

Exp.  1.  Strew  some  iron  filings  on  a  sheet  of  white  card  paper,  be- 
neath which  a  bar  magnet  has  been  placed  ;  occasionally  tap  the  paper 
to  facilitate  the  arrangement  of  the  filings.  The  beautiful  distribution 
of  the  filings  (as  exhibited  in  Fig.  6)  around  the  bar,  shows  the  manner 
in  which  the  attractive  force  of  the  different  points  in  the  bar  vary  — 
the  filings  are  most  accumulated  round  the  two  poles,  towards  which 
they  seem  to  converge  from  all  parts,  as  to  the  principal  centres  of  ac- 
tion :  on  the  other  hand,  the  central  portion  of  the  bar  scarcely  attracts 
any  of  the  iron  filings,  thereby  showing  that  the  centre  of  the  bar  is  a 
neutral  point ;  that  is  to  say,  it  does  not  possess  any  attractive  power. 
The  curves  formed  by  the  filings  are  known  by  the  name  of  the  mag- 
netic curves. 

This  experiment  furnishes  us  with  a  ready  method  of  detecting  the 
poles  of  a  natural  magnet. 

Exp.  2.  Take  a  magnetic  bar  N  S,  (Fig.  7,)  and  support  it  at  its  middle 
point  C ;  apply  at  any  number  of  equidistant  points  a,  b,  c,  c',  b't  &c., 


C'U 


Fig.  7. 

a  series  of  pieces  of  soft  iron  Avire ;  then  it  will  be  found  that  the  num- 
ber of  pieces  of  wire  which  the  magnet  can  support  will  increase  as  we 
approach  the  extremities  or  poles  N  and  S. 

The  centre  C  of  the  bar  has  been  called  the  neutral  point, 
or  point  of  magnetic  indifference,  and  the  poles  are  those  two 
points  where  the  greatest  attractive  force  is  found  to  reside, 
which  in  this  case  are  at  the  extremities.  The  term  pole  is 
sometimes  taken  to  mean  that  point  in  each  half  of  the  bar 
where  the  greatest  attractive  force  will  be  accumulated,  sup- 
posing the  magnet  to  be  acting  upon  a  piece  of  iron  or  steel 
placed  at  a  little  distance  from  it ;  in  this  case  the  poles  are, 
on  an  average,  at  the  distance  of  about  one  tenth  of  an  inch 
from  each  extremity. 


280          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

MAGNETIC  POLARITY. 

DIRECTIVE   PROPERTY    OF    THE   MAGNETIC    NEEDLE. 

3.  A  magnetized  steel  needle,  suspended  horizontally  by 
a  thread,  or  on  a  fine  point,  will  always  point  very  nearly 
north  and  south.  This  is  called  the  directive  polarity  of  the 
magnet.  This  direction  is  so  constant,*  that,  when  the  needle 
is  displaced,  it  returns  exactly  to  it,  after  a  few  vibrations. 
Moreover,  the  same  extremity  of  the  needle  always  points  to 
the  north,  and  the  same  extremity  to  the  south  ;  so  that  if  the 
needle  be  turned  half  way  round,  it  will  not  rest  until  it  has 
resumed  its  original  position.  The  extremity  which  points 
towards  the  north  is  called  the  north  pole  of  the  magnet,  and 
that  which  points  towards  the  south,  the  south  pole  of  the 
magnet.  This  remarkable  property  has  been  of  great  use  to 
navigators. 

Magnetic  needles  are  usually  con-    "==r  :=:^--II|2|i^^ 
structed  after  the  form  shown  in  -J^L, 


Fig.  8  ;  where  the  needle  turns  up-    r 
on  a  vertical  point,  which  enters  the 
conical  cap  screwed  into  the  centre 
of  the  needle. 

The  direction  in  which  the 
needle  points  has  been  called 
the  line  of  the  magnetic  merid- 
ian.    This  line   does   not  ex- 
actly coincide  with  the  direc- 
tion of  the   geographical  me- 
ridian, as   we   shall   hereafter  Fig^  g> 
more  fully  explain.     At  Lon- 
don, the  needle  at  present  points  about  24°  west  of  the  true 
north.     This  is   called  the  magnetic   variation,  or  magnetic 
declination.     This  declination  is  not  the  same  for  all  places 
on  the  earth,  and  it  is  continually  changing  for  all  places  on 
the  earth. 


MAGNETISM.  281 

Exp.  1.  Magnetize  a  small  sewing  needle;  place  the  needle  on  some 
water,  so  as  to  make  it  float :  after  a  little  time  the  needle  will  settle 
itself,  and  will  point  in  the  direction  of  north  and  south.  If  the  needle 
be  shifted  from  this  position,  it  will  return  to  the  same  position  again 
when  left  to  itself.  This  experiment  may  also  be  readily  performed  in 
the  following  manner :  — 

Exp.  2.   Take  a  strip  of  card  paper      -A. -w     p 

A  B  ;  suspend  it  upon  the  point  S  of   M< . ^.s  U     \ 

a  pin  passed  through  a  cork ;  place    ^~~ 
the  magnetized  needle  N  S  upon  one 
side  of  the  strip  of  card  paper;  re- 
store  the   balance  by  placing  some 
small  weight  W  upon  the  opposite  Fig.  9. 

side  of  the  card ;  then  the  card  will 
turn  round  until  it  points  north  and  south,  as  before  described. 

With  any  of  these  needles  the  following  experiments  may  be  per- 
formed, (excepting  the  cases  specified.) 

4.  Iron  or  steel  attracts  both  poles  of  the  needle. 

Exp.  3.  Hold  a  bit  of  iron  near  either  of  the  poles  of  the  needle  ;  the 
needle  will  follow  the  iron ;  by  moving  the  iron  round,  the  needle  will 
revolve  on  its  centre  in  the  same  direction. 

Exp.  4.  By  holding  a  bit  of  iron  near  to  the  sewing  needle  of  Exp.  1, 
it  may  be  made  to  float  about  in  any  direction. 

Exp.  5.  The  magnetic  swan.  —  This  philosophical  toy  consists  of  a 
piece  of  thin  sheet  iron,  made  into  the  shape  of  a  swan,  so  as  to  float 
upon  water. 

When  the  point  of  a  magnet  is  presented  to  the  swan,  it  appears  to 
swim  towards  the  point. 

5.  The  like  poles  of  magnets  repel  one  another,  and  the 
unlike  poles  attract.     This  law  of  magnetism  is  exactly,  anal- 
ogous to  the  law  of  attraction  and  repulsion  of  the  two  kinds 
of  electricity. 

In  order  to  distinguish  these  opposite  influences,  the  mag- 
netic principle  of  the  north  pole  is  called  positive  magnetism, 
or  -)->  and  that  of  the  south  pole,  negative  magnetism,  or  — . 

Exp.  6.  Bring  the  north  pole  of  a  magnet  near  to  the  south  pole  of 
the  needle,  and  it  will  be  attracted.  Bring  the  north  pole  of  a  magnet 
near  to  the  north  pole  of  the  needle,  and  it  will  be  repelled ;  and  so  on. 

This  always  enables  us  very  readily  to  ascertain  the  particular  poles 
24* 


282          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

of  a  magnet,  or  to  determine  whether  or  not  a  metal  bar  possesses  mag- 
netism ;  for  the  extremity  of  the  bar  which  attracts  the  north  pole  of  a 
needle  will  be  the  south  pole  of  the  bar,  and  the  other  extremity  will  be 
the  north  pole. 

Exp.  7.  Hang  a  small  key  to  the  north  pole  of  a  magnet ;  present  the 
south  pole  of  another  magnet  to  the  upper  extremity  of  the  key :  the 
key  will  instantly  fall.  Here  the  two  different  kinds  of  magnetism  neu- 
tralize each  other's  effects. 

•Exp.  8.  Immerse  the  like  poles  of  two  magnets  into  some  iron  filings  ; 
bring  the  two  poles  together,  and  the  filings  will  fall.  But  if  the  poles 
are  unlike,  the  filings  will  move  towards  each  other. 

Exp.  9.  Balance  a  bar  magnet  upon  a  common  pair  of  scales ;  bring 
the  pole  of  another  magnet  immediately  beneath  one  of  the  poles  of  the 
magnet  placed  on  the  scale  ;  then,  when  the  poles,  thus  brought  near  to 
each  other,  are  of  the  same  kind,  the  scale  will  ascend  from  the  repulsion 
of  the  magnets  ;  and,  on  the  contrary,  the  scale  will  descend  when  the 
poles  are  of  different  kinds. 

G.  If  a  magnet  be  broken,  each  part  becomes  a  perfect 
magnet. 

Exp.  10.  Break  a  magnetized  knitting  needle ;  test  the  polarity  ef 
each  end  of  the  pieces ;  the  poles  of  the  two  magnets  will  lie  in  the  same 
direction  as  the  poles  of  the  original  magnet. 


THEORY    OP   MAGNETISM. 

7.  The  theory  of  magnetism  is  exactly  analogous  to  the 
theory  of  electricity.  The  magnetic  fluid,  in  its  quiescent 
state,  is  supposed  to  consist  of  two  distinct  fluids  —  the  one 
being  the  north  or  positive  magnetism,  the  other  the  south  or 
negative  magnetism.  When  these  two  fluids  are  combined, 
they  form  the  magnetic  fluid  as  it  exists  in  non-magnetized 
substances,  or  substances  in  a  neutral  state.  The  particles  of 
the  same  kind  of  magnetism  repel  each  other ;  but  the  parti- 
cles of  opposite  kinds  of  magnetism  attract  each  other.  When 
the  two  fluids  exist  in  a  body  so  as  to  neutralize  each  other, 
then  the  body  exhibits  no  magnetism ;  but  if  this  state  of 
equilibrium  be  disturbed  by  any  cause,  then  the  magnetic 
state  is  induced. 


MAGNETISM.  283 

Fig.  10  gives  a  visible  representation  of  this  supposed  distribution  of 
the  particles  of  the  two  magnetic  fluids  in  the  body  of  a  magnetic  bar. 


Fig.  10. 

Here  we  suppose  the  light  squares  to  represent  the  particles  of  the  posi- 
tive fluid,  and  the  dark  squares  the  particles  of  the  negative  fluid.  As 
the  particles  of  the  two  fluids  are  separated  from  one  another,  they  must 
arrange  themselves  according  to  the  law  of  attraction  and  repulsion 
assumed  in  the  theory  ;  that  is  to  say,  a  positive  and  a  negative  particle 
must  always  be  contiguous  to  each  other.  From  this  it  follows  that  the 
extremity  N  will  be  a  north  pole,  and  S  a  south  pole. 

This  theory  readily  enables  us  to  explain  all  the  phenomena 
of  magnetism.  Let  us  take  a  few  examples :  - — 

When  the  extremity  of  a  bar  of  soft  iron  is  placed  in 
contact  with  the  north  pole  of  a  magnet,  the  opposite  extrem- 
ity of  the  bar  also  exhibits  north  or  positive  magnetism ;  this 
takes  place  in  consequence  of  the  repulsion  of  the  positive 
fluid  from,  and  the  attraction  of  the  negative  fluid  to,  the  north 
pole  of  the  magnet. 

When  a  magnetic  needle  is  broken,  it  is  obvious  that  the 
arrangement  of  the  particles  of  the  two  fluids  must  remain 
unchanged ;  that  is  to  say,  the  poles  in  the  two  magnets  must 
lie  in  the  same  direction  as  the  poles  of  the  original  magnet. 

When  the  north  pole  of  a  magnet  attracts  a  piece  of 
iron  wire,  the  extremity  of  the  wire  next  to  the  north  or  pos- 
itive pole  of  the  magnet  becomes  a  south  or  negative  pole, 
owing  to  the  repellent  action  exerted  on  the  positive  fluid,  and 
the  attractive  action  on  the  negative  fluid  of  the  wire,  by  the 
positive  fluid  of  the  magnetic  bar ;  hence  the  magnet  attracts 
the  wire  according  to  the  law  that  bodies  magnetized  with 
different  fluids  attract  each  other.  This  also  explains  the 
great  law  of  magnetic  induction,  which  we  shall  shortly  con- 
sider. 


284          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

The  like  poles  of  two  magnets  repel  each  other  by  vir- 
tue of  the  mutual  repulsion  subsisting  between  the  particles 
of  the  same  kind  of  magnetic  fluid ;  and  the  unlike  poles  of 
two  magnets  attract  each  other,  in  consequence  of  the  mutual 
attraction  subsisting  between  the  particles  of  the  two  different 
kinds  of  the  magnetic  fluid. 

The  north  pole  of  the  needle  is  directed  towards  the 
north  pole  of  the  earth,  because  the  earth  itself  is  a  great 
magnet,  having  its  negative  magnetic  pole  lying  towards  its 
north  geographical  pole,  and  its  positive  magnetic  pole  lying 
towards  its  south  geographical  pole. 

The  dip  of  the  magnetic  needle  may  be  readily  explained 
by  considering  the  dipping  direction  of  the  needle  to  be  the 
direction  of  the  resultant  of  the  magnetic  forces  residing  in 
the  earth,  which  act  upon  the  needle.  But  this  subject  will 
be  hereafter  more  fully  explained. 

8.  The  attractive  force  of  magnets  decreases  with  the 
distance. 

Exp.  11.  Place  the  south  pole  of  a  magnet  at  a  distance  from  the 
north  pole  of  the  needle,  and  a  little  to  the  right  or  left  of  it :  then  the 
needle  will  be  deflected  a  little  from  its  north  and  south  direction  ;  now 
bring  the  magnet  a  little  nearer  to  the  needle,  and  its  deflection  will  be 
increased,  and  so  on  —  thereby  showing  that  the  attractive  force  of  the 
magnet  increases  as  we  decrease  the  distance. 

It  will  also  be  observed  that  the  needle  vibrates  more  and  more  rapidly 
as  the  magnetic  bar  is  brought  more  closely  to  it.  Now,  the  rapidity  of 
these  vibrations  obviously  depends  upon  the  amount  of  the  magnetic 
force. 

The  law  of  the  attractive  force  of  a  magnet,  with  respect 
to  distance,  is  the  same  as  the  law  of  gravitation ;  that  is  to 
say,  the  attractive  force  of  a  magnet  varies  inversely  as  the 
squares  of  the  distance. 


MAGNETISM.  285 


MAGNETIC   INDUCTION  AND   CONDUCTION. 

9.  When  a  wire  of  soft  iron  is  placed  in  contact  with  the 
pole  of  a  magnet,  it  becomes,  as  it  were,  a  part  of  the  magnet 
itself;  for  every  portion  of  the  wire  has  the  same  polarity  as 
the  extremity  of  the  magnet  with  which  it  is  in  contact.  This 
may  be  called  magnetic  conduction.  But  if  the  contact  be 
ever  so  slightly  broken,  the  wire  becomes  a  complete  magnet 
having  two  poles ;  and  this  takes  place  in  consequence  of  the 
operation  of  another  principle,  —  that  of  induction,  —  which 
now  claims  our  attention.  When  the  soft  iron  wire  has  been 
entirely  removed  from  the  magnet,  after  a  short  time  it  no 
longer  possesses  any  magnetic  properties ;  it,  in  fact,  was  only 
decidedly  magnetic  while  it  was  in  contact  with  or  very  near 
to  the  magnetized  bar.  Soft  iron  receives  the  magnetic  influ- 
ence most  easily ;  but  it  also  parts  with  it  most  easily,  when 
taken  away  from  the  magnet.  Steel  and  cast  iron  are.  not  so 
easily  magnetized ;  but  when  the  magnetic  property  is  once 
imparted  to  them,  they  retain  it  for  years,  unless  they  are 
subject  to  some  counteracting  influence. 

Magnetic  induction  is  that  influence  which  a  magnet  exerts 
upon  substances  at  a  distance  from  it. 

Let  N  S  be  a  magnetic  bar,  N  being  its  north  pole,  and  S  its  south 
pole ;  n  s  a  soft  iron  bar,  having  its  extremity  s  placed  near  to  the  ex- 
tremity N  of  the  magnet ;  then  the  soft  iron  bar  n  s  will  be  a  perfect 
magnet  so  long  as  the  pole  of  the  magnet  N  S  is  near  to  its  extremity  s  ; 


Fig.  11. 

the  extremity  n,  in  fact,  will  be  its  north  pole,  and  s  its  south  pole.  To 
render  the  magnetic  induction  apparent,  a  small  key  may  be  suspended 
from  the  extremity  n.  The  nearer  the  bar  N  S  is  brought  to  the  bar  s  n, 
the  more  powerful  will  be  the  magnetism  induced  in  it.  Let  the  magnet 


286          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

N  S  be  taken  away ;  then,  after  a  short  time,  the  little  key  k  will  fall  off 
the  bar  s  n,  and  it  will  soon  lose  all  traces  of  magnetism. 

Here  the  positive  fluid  at  N  repels  the  positive  fluid  from  the  extremity 
s,  and  at  the  same  time  attracts  the  negative  fluid ;  hence  the  equilibrium 
of  the  two  fluids  in  the  soft  iron  n  s  is  disturbed,  the  extremity  s  being 
in  a  negative  magnetic  state,  and  the  extremity  n  in  a  positive  magnetic 
state ;  or,  in  other  words,  s  becomes  a  south  magnetic  pole,  and  n  a  north 
magnetic  pole. 

Bring  the  south  pole  of  a  bar  near  to  n  :  then  the  magnetic  induction 
will  be  doubled ;  the  lower  extremity  of  the  little  key  k  will  rise  towards 
this  south  pole  ;  and  a  much  heavier  key  may  be  supported  by  the  ex- 
tremity n.  Now  bring  the  north  pole  of  a  bar  near  to  n  :  then  the  key 
k  will  instantly  drop  off;  in  this  case,  the  two  poles,  being  of  the  same 
kind,  counteract  each  other's  influence. 

A  series  of  soft  iron  bars  may  be  magnetized  in  the  same  manner. 
Thus,  let  A  be  a  strong  magnetic  bar ;  B,  C,  and  D  a  series  of  soft  iron 


A  BCD 

Fig.  12. 

bars  placed  near  each  other,  as  shown  in  Fig.  12  :  then  all  these  soft  iron 
bars,  from  the  action  of  induction,  will  become  perfect  magnets,  having 
their  poles  as  indicated  by  the  letters  of  the  figure. 

The  law  of  magnetic  induction  is  exactly  analogous  to  the 
law  of  electrical  induction. 

The  following  simple  experiments  will  render  the  law  of  magnetic 
induction  and  conduction  more  apparent. 

MAGNETISM   BY    CONTACT. 

10.  Exp.  1.  Place  a  long  piece  of  soft  iron  wire  in  contact  with  the 
north  pole  of  a  powerful  magnet ;  test  the  magnetism  of  the  wire  by 
means  of  a  magnetic  needle  ;  the  south  pole  of  the  needle  will  be  every 
where  attracted  by  the  wire,  thereby  showing  that  the  wire  possesses 
north  polar  magnetism. 

Exp.  2.  Cut  some  short  pieces  of  iron  wire ;  present  the  end  of  one 
of  them  to  the  pole  of  a  strong  magnet :  it  will  be  immediately  attract- 
ed ;  the  free  end  of  this  wire  will  now  attract  a  second  wire,  and  this  in 
its  turn  will  attract  a  third  wire,  and  so  on.  All  these  wires  become 


MAGNETISM. 


287 


little  temporary  magnets,  owing  to  their  connection  \vith  the  pole  of  the 
magnetic  bar.  In  like  manner  the  phenomena  of  the  iron  filings  adher- 
ing to  the  pole  of  a  magnet  may  be  explained :  each  filing,  thus  sus- 
pended, is  converted  into  a  little  magnet. 

MAGNETISM   BY    INDUCTION. 

11.  Exp.  1.  Place  the  extremity  of  a  long  iron  wire  opposite  to  the 
north  pole  of  a  magnetic  needle  ;  bring  the  north  pole  of  a  magnetic  bar 
near  to  the  opposite  extremity  of  this  wire  :  the  needle  will  be  instantly 
repelled. 

Exp.  2.  Suspend  two  pieces  of  soft  iron  by  a  thread,  as  shown  in  Fig. 
13  ;  bring  the  north  pole  of  a  magnet  close  to  the  lower  extremities  of 
the  wires :  the  wires  will  repel  each  other,  after  the  manner  shown  in 
the  figure. 


Fig.  13. 


Fig.  14. 


Exp.  3.  Hold  a  large  key  near  the  pole  of  a  powerful  magnet :  then, 
as  the  key  becomes  a  magnet  by  induction,  it  will  carry  two  small  keys, 
one  at  its  lower  extremity,  and  the  other  at  its  upper  extremity,  as  shown 
in  Fig.  14. 


THE   DIP    OF   THE   MAGNETIC    NEEDLE. 

12.  Besides  the  directive  property,  the  magnetic  needle, 
when  freely  suspended,  has  another  remarkable  property, 
called  its  dip,  whereby  its  north  pole  dips  towards  the  north 
pole  of  the  earth  in  our  hemisphere,  and  its  south  pole  towards 
the  south  pole  of  the  earth  in  the  southern  hemisphere. 


288 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


At  present,  the  magnetic  dip  at  London  is  about  67°. 
This  property  may  be  readily  verified   in   the  following 
manner :  — 

Experiment.  Thrust  a  knitting  needle  n  s  through  a  cork  c,  as  shown 
in  Fig.  15  ;  at  right  angles  to  this  needle  thrust  a  fine  sewing  needle 
through  the  cork,  which  will  form  the  axis  of  the  needle  n  s  ;  attach  an 
untwisted  thread  a  d  b  to  the  axis,  and  suspend  the  whole  by  the  extrem- 
ity of  the  thread,  taking  care  to  thrust  n  s  either  one  way  or  the  other, 
until  it  is  suspended  in  a  perfectly  horizontal  position.  Now  magnetize 
the  needle  n  s,  which  may  readily  be  done  by  simply  keeping  it  for  a 
short  time  across  the  two  poles  of  a  horseshoe  magnet.  Again  suspend 
the  needle,  and  it  will  be  found  that  its  north  pole  will  dip  towards  the 
north.  Care  must  be  taken,  in  magnetizing  the  needle,  not  to  disturb 
the  axis. 


Fig.  15. 


Fig.  16. 


This  experiment  may  be  performed  with  more  precision  by  placing  the 
axis  a  a  between  two  upright  supports  a  b,  a  b,  as  shown  in  Fig.  16. 
The  best  supports  for  the  axis  are  the  edges  of  two  wine  glasses. 

The  angle  which  the  dipping  needle  makes  with  the  hori- 
zon at  any  place  is  called  the  angle  of  the  needle's  dip  at  that 
particular  place.  This  angle  is  not  the  same  for  all  places. 
At  places  in  the  northern  hemisphere,  the  north  pole  of  the 
needle  is  depressed ;  and  at  places  in  the  southern  hemisphere, 
the  south  pole  of  the  needle  is  depressed.  At  places  near  to 
the  equator,  the  needle  has  no  dip  —  that  is  to  say,  it  hangs 
horizontally. 


MAGNETISM. 


289 


Instruments  constructed  for  the  purpose  of  exactly  observing  the  dip 
have  a  vertical  graduated  circle  connected  with  them,  and  also  a  screw 
adjustment  for  placing  the  axis  exactly  horizontal,  as  shown  in  Fig.  17. 


Fig.  17. 


Fig.  18  represents  a  simple  form  of  magnetic  apparatus  for  showing 
the  direction  of  the  needle,  as  well  as  its  dip.  By  this  contrivance,  the 
needle  n  s  has  a  twofold  free  motion,  viz.,  a  free  motion  with  respect  to 
its  directive  property,  and  a  free  motion,  on  its  horizontal  axis  f  f,  with 
respect  to  the  angle  of  its  dip.  a  b  c  d  is  a  light  frame  suspended  by  an 
untwisted  thread;  the  horizontal  axis  ff  of  the  needle  turns  freely  in 
the  sides  a  b  and  d  c  of  the  frame.  A  needle,  thus  suspended,  will  settle 
itself  in  the  plane  of  the  magnetic  meridian,  and  will  also  assume  the 
true  angle  of  the  dip. 

The  subject  of  magnetic  variations,  &c.,  will  be  more  fully  explained 
in  connection  with  that  of  terrestrial  magnetism. 
25 


290 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


TO  MAGNETIZE   STEEL   BARS,  &c. 

I.      TO    MAGNETIZE    A   NEEDLE    WITHOUT    USING  AN 
ARTIFICIAL    MAGNET. 


13.  Fix  the  needle,  against  the  edge 
of  a  table,  in  the  magnetic  meridian  — 
that  is,  nearly  north  and  south  ;  hold 
a  long  poker  above  the  needle,  and 
another  one  below  it,  as  shown  in  Fig. 
19  ;  then  move  the  pokers  in  contrary 
directions  until  they  come  to  the  posi- 
tions shown  in  Fig.  20 ;  repeat  this 
operation  for  several  times,  always  ob- 
serving, at  every  successive  operation, 
to  move  the  pokers  in  the  same  man- 
ner, and  the  needle  will  be  magnet- 
ized. Here  the  pokers,  being  held  in 
the  direction  of  the  magnetic  dip,  real- 
ly become  magnets.  (See  the  subject 
of  Terrestrial  Magnetism.) 

Fig.  19. 


Fig.  20. 


II.      TO    MAGNETIZE    STEEL    BARS,   &c.,   BY   MAGNETS. 

14.  There  have  been  various  processes  devised  for  magnetizing  steel 
bars.  The  following  are  amongst  the  most  simple  and  efficient :  — 

Most  easy  Methods  of  magnetizing  a  small  Needle. 

Exp.  1.  Bring  the  pointed  extremity  of  a  sewing  needle  in  contact 
with  the  south  pole  of  a  magnet ;  let  the  needle  remain  in  contact  for  a 
few  minutes  ;  on  separating  them,  you  will  find  that  the  pointed  ex- 
tremity has  become  a  north  pole,  and  the  other  a  south  pole. 

Here  it  will  be  observed  thatrthe  end  of  the  needle  in  contact  with  the 
pole  of  the  magnet  acquires  an  opposite  or  dissimilar  magnetism  to  that 
of  the  pole.  The  equilibrium  of  the  two  magnetic  fluids  in  the  needle 
is  disturbed  by  the  pole  of  the  magnet  at  the  point  of  the  needle,  the 
dissimilar  magnetic  fluid  is  attracted  by  the  pole,  and  the  similar  fluid 
is  repelled. 

Exp.  2.  Rub  one  end  of  the  needle,  in  the  same  direction,  across  the 
north  pole  of  the  magnetic  bar,  and  then  rub  the  other  extremity  of  the 
needle  across  the  south  pole  of  the  bar  :  then  the  former  extremity  of 


MAGNETISM.  291 

the  needle  will  be  a  north  magnetic  pole,  and  the  other  extremity  a 
south  magnetic  pole. 

Exp.  3.  Place  the  needle  across  the  two  poles  of  a  horseshoe  magnet ; 
let  it  remain  there  for  some  time  ;  on  removing  it,  you  will  find  that  the 
extremity  in  contact  with  the  north  pole  of  the  magnet  has  become  a 
south  pole,  and  the  other  a  north  pole. 

Exp.  4.  Place  the  middle  of  a  needle  on  the  north  pole  of  a  magnet ; 
on  separating  them,  you  will  find  that  the  middle  of  the  needle  is  a 
south  pole,  and  that  its  extremities  are  north  poles.  This  will  form  a 
pretty  good  astatic  needle. 

Exp.  5.  With  the  pole  of  a  good  magnet,  draw  any  figure  upon  the 
surface  of  a  clear  steel  plate ;  sprinkle  iron  filings  upon  it :  the  filings 
will  remain  suspended  at  all  those  points  which  the  pole  of  the  magnet 
has  touched. 

Exp.  6.  Place  one  pole  of  a  magnet  in  the  middle  of  the  steel  bar ; 
draw  the  magnet  along  to  the  end  of  the  bar;  return  the  magnet, 
through  the  air,  to  the  middle  of  the  bar,  and  repeat  the  stroke  in  the 
same  direction  ;  repeat  this  operation  for  several  times.  Next  place  the 
other  pole  of  the  magnet  in  the  middle  of  the  steel  bar,  and  proceed  as 
before,  observing  that,  in  this  case,  the  magnet  must  be  drawn  to  the 
opposite  extremity  of  the  steel  bar.  , 

This  process  has  been  called  the  method  of  single  touch. 

The  Method  of  Double  Touch. 

15.  This  process  consists  in  touching  the  steel  bar  which  we  wish  to 
magnetize  with  both  poles  of  the  magnet  at  the  same  time.  This  method 
is  always  employed  when  large  steel  bars  are  to  be  magnetized. 

Fasten  two  bar  magnets  together,  so  that  their  dissimilar  poles  may  be 
about  one  eighth  of  an  inch  asunder ;  this  will  be  most  readily  effected 
by  inserting  a  piece  of  card  paper  between  them,  and  tying  them  with  a 
piece  of  cord.  Place  this  double  magnet  vertically  upon  the  middle  of 
the  steel  bar  ;  draw  the  magnet  to  the  end  of  the  bar ;  return  the  mag- 
net, through  the  air,  to  the  other  end  of  the  bar ;  draw  the  magnet,  as 
before,  to  the  opposite  end  ;  repeat  this  process  for  several  times,  taking 
care  to  keep  the  pole  of  the  compound  magnet  always  in  the  same  rela- 
tive position,  and  to  stop  the  process  when  the  magnet  has  arrived  at  the 
middle  of  the  bar.  The  operation  should  be  performed  on  both  sides 
of  the  bar. 

A  horseshoe  magnet,  having  its  poles  near  together,  will  answer  the 
same  purposes  as  the  double  magnet  just  described. 

This  method  may  be  employed  to  magnetize  two  or  more  bars  at  the 
same  tune. 


292  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Place  two  steel  bars,  N  S,  N  S,  of  the  same  size,  parallel  to  each 


Fig.  21. 

other,  and  connect  their  extremities  with  two  pieces  of  soft  iron,  A  and 
B.     (See  Fig.  21.) 

Place  the  pole  of  the  double  magnet  on  the  middle  of  one  of  the  steel 
bars,  and  move  it  completely  round  the  frame,  constantly  keeping  the 
poles  of  the  double  magnet  in  the  same  direction  ;  when  you  have  com- 
pleted about  a  dozen  revolutions,  turn  the  plates  and  proceed  as  before. 
The  poles  of  the  steel  bars  will  have  a  reverse  position  to  the  poles  of  the 
double  magnet. 

To  magnetize  Horseshoe  Bars. 

Fig.  22  shows  the  method  of  magnetizing  one  horseshoe  bar  N.  Place 
a  piece  of  soft  iron  K,  called  a  keeper,  across  the  extremities  of  the  horse- 
shoe ;  place  a  horseshoe  magnet  M,  whose  legs  are  at  the  same  distance 
apart  as  those  of  the  bar  N,  with  its  poles  perpendicular  to  the  keeper  K ; 


Fig.  22. 


Fig.  23. 


draw  the  magnet  towards  the  bent  part  of  the  horseshoe ;  when  it  has 
arrived  there,  lift  it  off,  and  bring  it  back  to  its  first  position  ;  repeat  the 
operation  for  about  a  dozen  times  ;  then  turn  the  horseshoe  bar,  with  its 
keeper  still  on,  and  repeat  the  operation  as  before  ;  and  so  on. 

The  polarity  of  each  leg  of  the  horseshoe  bar  will  be  similar  to  that 
of  the  leg  of  the  magnet  first  placed  in  contact  with  it. 

Fig.  23  shows  the  method  of  magnetizing  two  horseshoe  bars  at  the 
same  time.  The  bars  are  placed  with  their  extremities  in  contact ;  and 


MAGNETISM. 


293 


the  horseshoe  magnet  M  is  moved  from  the  curved  part  of  one  bar  to 
the  curved  part  of  the  other,  constantly  in  the  same  direction. 

The  following  is  also  a  convenient  and  efficient  mode  of  arrangement 
(see  Fig.  24)  for  magnetizing  bars. 


Fig.  24. 

M  M  is  the  horseshoe  magnet,  placed  with  its  poles  against  the  ex- 
tremities of  the  horseshoe  bar  to  be  magnetized ;  A  is  a  soft  iron  keeper 
extending  between  the  legs  of  the  horseshoes  ;  this  keeper,  or  feeder,  is 
drawn  in  the  same  way  as  the  magnet  represented  in  Fig.  23. 

In  the  same  manner,  straight  bars  may  be  magnetized. 

In  Fig.  25,  M  M  represents  the  magnet,  A  the  feeder,  B  B  the  two 
bars  to  be  magnetized,  and  K  their  keeper. 


Fig.  25. 
When  magnetic  bars  are  not  in  use,  they  should  always  be  put  away 


with  their  keepers  upon  them;  this  not  merely 
preserves  their  magnetism,  but  also  tends  to  in- 
crease it. 

A  compound  horseshoe  consists  of  a  number  of 
horseshoe  magnets  bound  together  by  screws,  and 
connected  at  their  poles  by  means  of  a  keeper,  as 
shown  in  Fig.  26. 

Fig.  27  represents  a  lot  of  bars  bound  together 
in  the  same  manner. 


Fig.  26.        Fig.  27. 


On  the  best   Quality  of  Steel  for  making  Magnets. 

16.  The  steel  best  suited  for  artificial  magnets  is  of  a  fine 
grain,  of  uniform  structure  throughout,  and  free  from  flaws. 
A  principal  requisite  is,  that  it  should  possess  a  proper  degree 
of  hardness,  and  that  it  should  be  equally  hardened,  through- 
out the  entire  mass ;  for  if  too  hard,  it  is  extremely  difficult 
25* 


294          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

to  impart  to  it  any  magnetic  virtue  ;  and  if  too  soft,  it  read- 
ily loses  it  when  given.  It  has  been  found  most  advantageous 
to  make  the  steel  in  the  first  instance  brittle,  like  glass,  and 
then  to  heat  it  a  second  time,  till  it  becomes  of  a  straw  or  vio- 
let color. 

The  capacity  and  tenacity  of  artificial  magnets  are  also 
affected  by  their  form  and  dimensions.  It  has  been  ascer- 
tained that  the  breadth  of  a  bar  magnet  should  be  about  one 
twentieth  of  its  length,  and  its  thickness  from  one  fourth  to 
one  third  of  its  breadth.  In  a  horseshoe  magnet,  the  space 
between  the  two  poles  ought  not  to  be  greater  than  the  thick-" 
ness  of  the  bar  of  which  the  magnet  consists.  Lastly,  it  is 
necessary  that  both  bar  and  horseshoe  magnets  be  well  pol- 
ished, and  that  their  faces  be  as  level  as  possible. 

Magnetism  is  readily  excited  in  soft  Iron  Bars. 

17.  A  bar  of  soft  iron,  placed  in  the  direction  of  the  mag- 
netic dip,  becomes  magnetic  from  the  inductive  influence  of 
the  earth  acting  like  a  magnet  upon  the  bar.  A  few  blows 
applied  at  one  extremity  of  the  bar,  thereby  causing  its  par- 
ticles to  vibrate,  will  generally  aid  the  inductive  influence  of 
the  earth. 

A  bar  of  iron  heated  to  redness,  and  allowed  to  cool  after 
being  placed  in  the  direction  of  the  magnetic  dip,  will  acquire 
a  certain  degree  of  magnetism.  Hence  pokers  and  iron  rails, 
which  have  been  kept  for  a  long  time  standing  in  a  somewhat 
vertical  position,  are  generally  found  to  possess  a  low  degree 
of  magnetism. 

A  piece  of  iron  wire  may  be  rendered  magnetic  by  twisting 
it  until  it  breaks ;  and,  in  like  manner,  files  and  gimlets, 
after  having  been  some  time  in  use,  become  so  much  magnet- 
ized as  to  attract  iron  filings. 

Voltaic  electricity  is  the  most  powerful  means  of  rendering 
bodies  magnetic. 


MAGNETISM. 


295 


Experiment.  Allow  a  magnetic  needle 
N  S  to  assume  its  north  and  south  direc- 
tion ;  take  a  non-magnetized  poker,  and 
hold  it  in  a  horizontal  position  and  at  right 
angles  to  the  direction  of  the  needle,  so  as 
to  bring  one  of  its  extremities,  say  its  lower 
extremity,  near  to  the  north  pole  of  the 
needle;  the  needle  will,  of  course,  be  at- 
tracted, if  the  poker  is  not  magnetic ;  now 
hold  the  poker  in  the  direction  of  the  mag- 
netic dip,  as  shown  in  Fig.  28,  and  the 
north  pole  N  will  be  repelled  —  thereby 
showing  that  the  lower  extremity  n  of  the 
poker  is  a  north  magnetic  pole.  The  ef- 
fect will  be  increased  by  striking  the  head  s 


68°  V 


Fig.  28. 
of  the  poker  with  a  hammer. 


TERRESTRIAL  MAGNETISM. 

18.  In  order  to  account  for  the  directive  and  dipping  prop- 
erties of  the  needle,  it  has  been  stated  that  we  must  regard 
the  earth  as  a  great  magnet,  having  a  negative  magnetic  pole 
lying  towards  the  north  geographical  pole,  and  a  positive  mag- 
netic pole  lying  somewhere  towards  the  south  geographical 
pole.  The  following  experiment  is  highly  calculated  to  illus- 
trate this  theory. 

Experiment.  Place  a  magnetic  needle  (see  Figs.  29  and  30)  n  s  over 
the  middle  part  A  of  a  magnetic  bar  N  S ;  in  this  position  the  needle  is 


b  A  B 

Fig.  29. 


Fig.  30. 


296  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

exactly  horizontal,  and  the  south  pole  of  the  needle  is  directed  to  the 
north  pole  of  the  magnet,  and  the  north  pole  of  the  needle  to  the  south 
pole  of  the  magnet.  Thus  we  can  assign  a  cause  for  the  directive  prop- 
erty of  the  needle.  Now  slowly  move  the  needle  along  the  bar  from 
A  to  S  ;  at  the  position  B  the  north  pole  of  the  needle  dips  towards  the 
south  pole  of  the  magnet ;  at  the  position  C,  the  north  pole  of  the  nee- 
dle dips  still  more  towards  the  south  pole  of  the  magnet ;  and  at  S  the 
needle  hangs  vertically,  with  its  north  pole  pointing  to  the  south  pole 
of  the  magnet.  Now,  in  like  manner,  move  the  needle  from  A  to  N  ; 
at  the  position  b  the  south  pole  of  the  needle  dips  towards  the  north  pole 
of  the  magnet ;  and  so  on  as  before.  (In  Fig.  30  the  needle  is  supposed 
to  be  suspended  by  a  thread.)  Thus  we  can  account  for  the  mag- 
netic dip. 

The  phenomena  of  the  direction  and  dip  of  magnetic  nee- 
dles at  different  parts  on  the  earth's  surface  are  found  to 
coincide  with  the  effects  which  a  bar  magnet  produces  on  the 
needle,  as  above  described ;  hence  we  are  led  to  conclude  that 
the  earth  is  a  great  bipolar  magnet,  whose  poles  lie  towards 
the  geographical  poles  of  the  earth.  As  like  poles  attract, 
and  unlike  poles  repel  each  other,  it  follows  that  the  mag- 
netic  pole  of  the  earth  lying  towards  the  north  is  a  negative 
magnetic  pole,  and  that  the  one  lying  towards  the  south  is  a 
positive  magnetic  pole.  The  former  magnetic  pole  is  situated 
in  North  America,  in  the  vicinity  of  Hudson's  Bay,  in  70°  5' 
N.  lat.,  and  114°  55'  W.  long.;  and  the  other  in  7.2°  35'  S. 
lat.,  and  152°  30'  E.  long.  At  these  places  the  dipping  nee- 
dle assumes  a  vertical  position,  as  shown  at  P  and  K,  Fig.  31. 
Sir  James  Ross  found  the  pole  P  in  the  northern  hemisphere 
during  his  arctic  expedition  of  1829.  The  actual  existence 
of  the  magnetic  poles  in  these  places  is  further  confirmed  by 
the  fact  that  the  magnetic  needle,  at  different  parts  on  the 
earth's  surface,  is  always  directed  towards  these  points  as 
magnetic  poles. 

At  the  magnetic  equator  M  T  the  needle  assumes  a  horizontal  position. 
As  we  approach  the  magnetic  pole  P,  the  north  pole  of  the  needle  dips 
more  and  more ;  and,  on  the  contrary,  as  we  approach  the  magnetic  pole 
K,  the  south  pole  of  the  needle  dips  more  and  more.  On  the  magnetic 
meridian  K  G  P,  K  V  P,  &c.,  the  needle  has  always  the  same  general 


ELECTRICITY. 


297 


direction,  although  it  varies  in  its  dip.    Let  N  S  represent  the  axis  of 
the  earth,  E  Q  its  geographical  equator,  S  V  N  a  geographical  meridian, 


K  V  P,  K  X  P,  magnetic  meridians ;  then  the  angles  P  V  N  and 
P  X  N  will  be  the  declinations,  or  angles  of  variation,  of  the  magnetic 
needle  at  the  points  V  and  X,  respectively.  The  commander  of  a  ship 
at  Y,  knowing  from  his  charts  the  deviation  of  the  needle  at  the  par- 
ticular spot,  will  be  able  to  ascertain  the  true  north  and  south.  The 
magnetic  parallels  D  F,  J  L,  &c.,  are  lines  of  equal  magnetic  dip,  as 
shown  at  r  and  t,  on  the  magnetic  parallel  D  F,  where  the  needles  5  n, 
s  n,  dip  towards  the  pole  P,  at  the  same  angle. 

It  must  be  borne  in  mind  that  these  different  magnetic  lines  upon  the 
earth  are  not  exactly  formed  by  true  sections  of  the  sphere,  like  the 
geographical  circles.  Indeed,  some  of  these  magnetic  lines  have  the 
shape  of  looped  curves,  or  curves  of  double  curvature,  differing  more  or 
less  from  the  circular  lines  shown  in  Fig.  31. 

The  lines  of  equal  dip  have  been  called  isoclinic  lines  ; 
these  lines,  as  we  have  shown,  surround  the  globe,  running 
nearly  parallel  with  the  magnetic  equator.  It  is  a  remarkable 
fact,  that  there  is  a  coincidence  subsisting  between  these  lines 
and  the  isothermal  lines,  or  lines  of  equal  heat,  upon  the 
globe :  this  coincidence  indicates  that  the  earth's  magnetism  is 
intimately  connected  with  terrestrial  heat. 


(    UNIVERSITY 
\ 


298  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

The  inductive  influence  of  the  earth  upon  bars  of  soft  iron  (see  exper- 
iments, p.  13  and  26,)  bears  a  striking  analogy  to  the  induction  of  mag- 
netism by  ordinary  magnetic  bars.  The  magnetic  eifects  of  the  earth 
are  undoubtedly  attributable  to  the  inductive  influence  of  terrestrial 
magnetism. 

VARIATIONS    OF   THE   NEEDLE. 

19.  The  earth's  magnetic  powers  are  subject  to  both  reg- 
ular and  irregular  variations.  These  variations  are  indi- 
cated by  the  changes  which  occur  at  the  same  place,  in  the 
declination  and  dip  of  the  needle,  and  in  its  magnetic  in- 
tensity. 

The  regular  variations  follow  a  certain  law*,  which  enables 
us  to  calculate  beforehand  the  changes  that  in  future  will  take 
place.  These  regular  variations  are  either  secular  or  peri- 
odic. The  secular  changes  become  only  evident  after  the 
lapse  of  years,  and  the  periodic  are  those  which,  as  it  were, 
oscillate  within  short  periods  of  time. 

Of  all  the  secular  variations,  the  declination  is  that  which 
has  been  most  observed,  and  which  has  been  most  exactly  de- 
termined. The  dip  and  intensity  have  but  recently  claimed 
the  attention  of  philosophers. 

About  the  year  1600,  the  needle  at  London  pointed'^0  to  the  east  of 
the  north ;  1660  it  pointed  due  north ;  from  which  time  it  gradually 
deviated  to  the  west  of  the  north  until  the  year  1818,  when  it  deviated 
24-3°  to  the  west  of  north,  which  was  its  maximum  deviation  ;  but  for 
the  last  30  years  its  declination  has  certainly  been  decreasing,  and  in  all 
probability  it  will  continue  to  do  so  until  it  again  becomes  due  north ; 
then  the  declination  will  increase  towards  the  east  until  the  needle  has 
again  attained  its  maximum  eastern  declination,  when  it  will  again 
return. 

All  that  is  known  with  certainty  relative  to  the  dip  of  the  needle  is, 
that  at  present  it  is  decreasing  in  Europe.  The  maximum  dip  of  the 
needle  at  London  took  place  about  a  century  ago,  when  it  was  about 
74°  ;  since  that  time  it  has  been  going  on  decreasing,  with  great  regu- 
larity, at  the  rate  of  3'  annually.  At  London,  the  dip  of  the  needle  at 
the  present  time  is  about  68°. 

The  variation  of  the  magnetic  intensity  has  but  recently  claimed  the 
attention  of  experimentalists  ;  however,  it  seems  highly  probable  that 
this  intensity  is  at  present  decreasing  in  Europe. 


MAGNETISM.  299 

The  compass  needle,  also,  undergoes  diurnal  and  annual  variations. 
These  variations  appear  to  be  intimately  connected  with  the  heat  of  the 
sun.  From  sunrise  to  a  little  after  noon,  the  north  pole  of  the  needle 
moves  towards  the  west,  and  after  that  time  it  retrogrades  towards  the 
east  until  a  little  after  sunset  in  the  evening,  when  it  remains  nearly 
stationary  until  sunrise.  The  extent  of  these  variations  depends,  not 
only  on  the  time  of  the  year,  but  also  upon  the  situation  of  the  place. 
In  London,  during  the  heat  of  summer,  the  variation  is  about  19', 
whereas,  in  whiter,  it  is  only  about  7'.  In  Paris,  the  summer  variation 
is  about  15',  and  in  winter  about  9'.  These  variations  disappear  under 
the  magnetic  equator ;  and  on  the  south  of  it  they  are  found  to  exist  in 
an  inverted  order. 

The  dip  of  the  needle  is  also  subject  to  daily  variations,  which  also 
appear  to  depend  upon  the  action  of  the  sun's  heat  upon  the  earth  ;  but 
they  do  not  exactly  accord  with  the  daily  variations  of  declination. 

The  variations  of  magnetic  intensity  also  appear  to  depend  upon  the 
sun's  heat. 

The  irregular  magnetic  variations  are  connected  with  certain  electrical 
and  meteoric  phenomena,  such  as  the  aurora  borealis,  lightning,  and 
even  volcanic  eruptions.  A  flash  of  lightning  has  been  known  to  reverse 
the  poles  of  a  needle,  and  even  to  destroy  its  magnetism  entirely. 

THE   DECLINATION    COMPASS    AND    MARINER'S    COMPASS. 

20.  This  apparatus  is  used  for  observing  and  measuring  the  declina- 
tion of  the  needle,  or,  conversely,  for  determining  the  north  and  south 
direction,  or  the  meridian  line,  when  the  magnetic  declination  is  known. 
It  consists  of  a  magnetic  needle  N  S  (see  Fig.  32)  delicately  suspended 
by  means  of  an  agate  or  steel  cap  O  resting  on  a  pivot.  E  F  is  a  grad- 
uated circle,  on  which  is  read  the  division  corresponding  to  the  position 
of  the  needle.  The  needle,  with  its  graduated  circle,  is  placed  in  a  cir- 
cular box  covered  with  glass.  The  instrument  is  usually  furnished  with 
a  telescope  A  B,  turning  on  a  horizontal  axis  C  D,  which  carries  an  air 
level  and  a  vertical  quadrant  A,  divided  to  measure  the  angles  described 
by  the  telescope.  The  box  is  capable  of  turning  round  on  a  vertical  axis, 
by  which  it  is  fixed  on  its  stand,  in  order  to  bring  the  telescope  in  the 
direction  of  the  meridian ;  then  the  angle  formed  by  the  direction  of  the 
telescope,  with  the  direction  of  the  needle,  gives  the  angle  of  declina- 
tion ;  or,  when  the  declination  is  known,  the  box  is  turned  until  the 
angle  made  by  the  axis  of  the  telescope  and  the  direction  of  the  needle 
are  equal  to  it ;  then  this  gives  the  position  of  the  meridian. 

The  marine  compass  differs  from  the  ordinary  compass  simply  in  hav- 
ing a  double  suspension,  which  admits  of  its  maintaining  itself  in  a  hor- 


300          NATURAL    AND    EXPERIMENTAL   PHILOSOPHY. 


Fig.  32. 

izontal  position,  notwithstanding  the  rolling  of  the  ship.    Fig.  33  rep- 
resents a  form  of  this  double  suspension ;  where  C  is  the  agate  or  steel 


cap  fixed  to  the  needle  N  S;  D  another  cap,  with  a  pivot  fixed  to  its 
upper  part,  on  which  cap  C  turns ;  A  the  pivot  on  which  the  cap  D 
turns. 


THE  ASTATIC    NEEDLE. 


21.   For  the  purpose  of  conducting  many  interesting  experiments,  it  is 
requisite  to  have  magnetic  needles  on  which  the  earth  does  not  exert  any 


^MAGNETISM.  301 

directing  influence  :  needles  of  this  sort  are  called  Astatic  needles.  This 
object  is  readily  attained  by  fixing  two  equal  needles  to  a  common  point 
of  suspension,  with  their  contrary  poles  together.  By  this  means,  the 
one  needle  exactly  counteracts  the  directive  tendency  of  the  other,  so 
that  the  compound  or  astatic  needle  will  be  free  to  obey  the  slightest 
attractive  force,  without  being  influenced  by  the  magnetic  power  of  the 
earth. 

Fig.  33«  represents  a  simple  and  highly  ser-  c 

viceable  astatic  needle  ;  s  n  and  n  s  are  two  mag- 
netic needles,  of  the  same  size  and  magnetic  in- 
tensity, connected  at  their  centres  by  a  wire  a  b  ; 
the  astatic  needle  thus  formed  is  suspended  by  a 
fine  thread  of  untwisted  silk  a  c.  The  applica-  ^^^^ 


tion  of  this  astatic  needle  will  be  noticed  in  b 

connection  with  the  subject  of  electro- dynamics.  Fig.  33a. 

The  Inclination  Compass. 

This  apparatus  is  used  for  observing  and  measuring  the  dip  of  the 
needle  at  different  places  on  the  earth's  surface,  or  at  different  periods  of 
time  at  any  place.  (See  Fig.  17,  p.  289.) 


AMPERE'S    THEORY    OF    MAGNETISM   AND    ELECTRO- 
DYNAMICS. 

22.  Ampere  considered  that  a  magnet  is  formed  by  a  magnetic  cur- 
rent, which  he  believed  to  be  the  same  as  an  electric  current,  circulating 
round  it  constantly  in  the  same  direction,  as 
shown  in  Fig.  34.  Supposing  the  magnet  to 
have  its  north  and  south  direction,  then  the  cur- 
rent enters  at  the  south  poles,  and  circulates 
round  the  magnet  spirally,  (like  a  corkscrew,) 
along  its  length  from  south  to  north,  as  shown 

in  the  figure  ;  that  is,  the  current  is  directed  from  east  to  west  in  the 
lower  face  of  the  magnet,  and  therefore  from  west  to  east  in  its  upper 
face ;  or,  in  other  words,  the  current  is  ascending  in  the  face  situated  on 
the  west,  and  descending  in  the  face  on  the  east.  Steel  bars  become 
magnets  when  this  regular  current  is  permanently  excited  in  them. 
Ampere  established  this  theory  by  showing  that  a  helix  of  copper  wire, 
26 


302  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

through  which  an  electric  current  is  transmitted,  possesses  all  the  prop- 
erties of  a  magnetic  needle.  As  a  necessary  consequence  of  this  theory, 
it  follows  that  parallel  currents  moving  in.  the  same  direction  mutually 
attract,  and  that  they  mutually  repel  when  they  are  moving  in  a  con- 
trary direction.  Now,  wires  conducting  electrical  currents  have  really 
this  property.  This  explains  why  like  poles  repel  and  unlike  poles 
attract.  But  this  theory  will  be  more  fully  explained  in  connection 
with  the  subject  of  electro- dynamics. 


VOLTAIC  ELECTRICITY. 

1.  GALVANISM,  or  Voltaic  Electricity,  is  produced  by  a 
certain  chemical  action  upon  two  different  metals  when  brought 
into  contact.  Galvani,  of  Bologna,  observed  that  when  he 
touched  a  nerve  and  muscle  in  the  leg  of  a  dead  frog  with 
two  different  metals,  on  bringing 
these  metals  into  contact,  the 
leg  underwent  a  convulsive  mo- 
tion, as  shown  in  Fig.  35,  where 
Z  and  C  are  the  two  metals 
brought  into  contact  at  A,  the 
extremity  B  being  in  contact 
with  the  nerve,  and  D  with  the 
muscle.  Galvani  considered  this 
effect  as  due  to  somethin^in  the 

animal  structure,  and  hence  he  called  it  animal  electricity,  but, 
out  of  respect  to  the  discoverer,  the  name  of  galvanic  elec- 
tricity was  given  to  it.  But  Volta  soon  after  showed  that  the 
effect  was  entirely  due  to  the  production  of  electricity  by  the 
action  of  the  two  metals  upon  each  other,  and  that  the  nerves 
and  muscles  of  the  animal  merely  exhibited  the  free  elec- 
tricity in  the  same  way  as  any  other  delicate  electroscope 
might  do.  This  leading  conception  conducted  him  to  a  series 
of  splendid  discoveries,  and  in  particular,  in  the  first  year  of 
the  present  century,  led  him  to  the  construction  of  the  voltaic 
pile,  which  stands  in  the  same  relation  to  voltaic  electricity 
that  the  common  electrical  machine  does  to  frictional  elec- 
tricity. 

VOLTAIC    PILE. 

2.    A  number  of  circular  plates  of  copper  and  zinc,  and  of  cloth  or 
card,  about  3  inches  diameter,  were  provided,  and  arranged  in  the  form 

(303) 


304 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


of  a  pile,  (Fig.  36.)  The  base  of  the  pile  is  a  copper  disk,  upon  which  a 
zinc  disk  is  placed  ;  (these  two  disks  form  \vhat  is  called  a  pair;)  over 
this  pair  a  second  similar  pair  is  placed,  observing  always  that  the  copper 
is  below  the  zinc ;  the  second  pair  is  separated  from  the 
first  by  the  circular  cloth  or  card,  moistened  with  a 
weak  saline  or  acid  solution.  Upon  the  second  pair  is 
placed  a  third,  separated  also  by  a  moistened  circular 
piece  of  cloth  or  card,  similar  to  that  which  preceded. 
In  this  manner  a  considerable  number  of  pairs  are 
placed  in  the  same  order,  one  over  the  other,  and  re- 
tained in  their  upright  position  by  means  of  rods  of 
glass.  When  the  base  plate  of  the  pile  rests  upon  an 
insulating  plate  of  glass,  this  lower  plate  is  found  to  be 
charged  with  negative  electricity,  whilst  its  upper  plate 
is  charged  with  positive  electricity.  These  extremities 
are  called  the  poles  of  the  pile  or  battery,  the  lower  ex- 
tremity being  the  negative  pole,  and  the  upper  extrem- 
ity  the  positive  pole.  If  the  metals  had  been  placed  in 
a  reverse  order,  then  the  poles  would  also  obviously  be  reversed.  Two 
wires,  one  leading  from  the  extreme  copper  plate,  and  the  other  from  the 
extreme  zinc  plate,  conduct  the  electricity  of  the  respective  poles  to  any 
substance  upon  which  the  electric  fluid  is  required  to  act.  When  the 
extremities  of  these  wires  are  brought  together,  an  electric  spark  passes 
between  them,  arising  from  the  neutralization  of  the  two  different  kinds 
of  electricity.  When  these  wires  are  held  one  in  each  hand,  (the  num- 
ber of  pairs  in  the  pile  being  sufficiently  great,)  a  rapid  succession  of 
shocks  are  felt.  When  the  extremities  of  the  two  wires  'are  connected 
by  a  very  fine  platinum  or  silver  wire  about  half  an  inch  in  length,  the 
neutralization  of  the  two  electricities  causes  this  fine  wire  to  rise  in  tem- 
perature, and  to  become  red  hot.  The  length 
of  the  fine  wire  which  may  thus  be  rendered 
incandescent  is  in  proportion  to  the  power  of 
the  pile. 

When  the  two  wires  proceeding  from  the  two 
poles  of  the  pile  are  immersed  near  each  other 
in  acidulated  wrater,  the  water  is  decomposed 
into  its  two  constituent  gases,  hydrogen  and 
oxygen,  the  oxygen  being  liberated  from  the 
wire  proceeding  from  the  positive  pole,  and  the 
hydrogen  from  the  wire  proceeding  from  the 
negative  pole;  the  volumes  of  the  gases  are 
constantly  in  the  same  proportions  that  consti- 
tute water  ;  that  is  to  say,  one  volume  of  oxy- 


Fig.  37. 


VOLTAIC    ELECTRICITY. 


305 


gen  to  two  volumes  of  hydrogen,  as  shown  in  Fig.  37.     In  this  experi- 
ment, the  submerged  parts  of  the  two  wires  must  be  platinum. 

These  phenomena  are  merely  simple  examples  of  the  various  and  im- 
portant effects  produced  by  the  action  of  the  voltaic  pile  or  battery, 
which  we  shall  hereafter  more  fully  consider. 

When  the  number  of  pairs  in  the  pile  is  so 
great  that  its  height  would  be  inconvenient 
when  placed  in  a  single  column,  the  plates 
may  be  arranged  in  two  or  more  columns,  as 
shown  in  Fig.  38,  where  the  continuity  of  the 
pile  is  sustained  by  the  bars  B  and  B'.  In 
this  case,  the  negative  pole  of  the  pile  is  at 
N,  and  the  positive  pole  at  P,  and  the  effect 
of  the  whole  is  the  same  as  if  the  second  were 
placed  over  the  first,  and  the  third  over  the 
second. 

Volta  proposed  a  second  arrangement  of  the 
pile  or  battery,  called  the  Couronne  de  Tasses. 

This  form  of  the  apparatus  is  represented  in  Fig.  38. 

Fig.  39  ;    it  consists  of  a  series  of  cups  or 

glasses,  containing  a  saline  or  acidulated  solution.     Each  pair  of  copper 
and  zinc  plates  is  immersed  in  the  separate  cups ;  the  zinc  plate  in  one 


Fig.  39. 

cup  being  connected  by  a  wire  with  the  copper  plate  in  the  succeeding 
cup,  and  so  on  ;  the  wire  Q,  proceeding  from  the  first  zinc  plate,  forms 
the  negative  pole  of  the  battery,  and  the  wire  P,  proceeding  from  the 
last  copper  plate,  forms  the  positive  pole  of  the  battery ;  the  same,  in 
matter  of  fact,  as  in  the  ordinary  pile  just  described. 

Very  great  improvements  have  been  made  in  the  construction  of  these 
batteries ;  but  before  we  proceed  to  describe  them,  we  shall  give  a  few 
simple  and  instructive  experiments,  calculated  to  elucidate  the  general 
principles  and  effects  of  voltaic  electricity. 
26* 


306  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

PRELIMINARY   VIEWS    AND    SIMPLE    EXPERIMENTS     ON   VOL- 
TAIC   ELECTRICITY. 

3.  Exp.  1.  Place  a  piece  of  sheet  zinc  under  your  tongue ;  lay  a  half 
crown  upon  the  tongue  ;  no  peculiar  sensation  is  felt  so  long  as  the 
two  metals  do  not  touch  each  other  :  now  bring  the  edges  of  the  two 
metals  in  contact  with  each  other ;  a  disagreeable  taste,  something  like 
copperas,  is  instantly  excited. 

Here  the  saliva  on  the  tongue  between  the  two  metals  is  the  exciting 
cause  of  the  development  of  the  electric  fluid ;  and  when  the  edges  of 
the  metals  are  brought  into  contact,  the  voltaic  circle  is  formed,  and  the 
peculiar  sensation  of  taste  is  the  effect ;  but  when  the  voltaic  circle  is 
broken  this  sensation  instantly  ceases.  The  peculiar  taste  of  porter, 
when  drunk  out  of  a  pewter  pot,  is  also  due  to  the  same  cause. 

Exp.  2.  Instead  of  the  half  crown,  in  the  last  experiment,  use  a  piece 
of  charcoal  or  a  piece  of  cast  iron. 

Exp.  3.  The  first  experiment  gave  you  a  taste  of  voltaic  electricity  ; 
now  the  following  experiment  will  give  you  a  sight  of  it. 

Place  a  silver  spoon  between  the  gums  and  one  cheek,  and  a  strip  of 
zinc,  in  a  similar  position,  on  the  other  cheek  ;  complete  the  voltaic  cir- 
cuit by  bringing  the  extremities  of  the  metals  together  on  the  outside  of 
the  mouth  ;  a  slight  flash  of  electric  light  will  instantly  be  seen.  Repeat 
the  experiment :  the  flash  will  always  be  seen  at  the  instant  the  two 
metals  are  brought  into  contact ;  and  a  smaller  flash  will  be  seen  at  the 
instant  the  contact  is  broken.  The  first  experiment  maybe  also  per- 
formed by  the  silver  tea  spoon  and  the  zinc  strip. 

Exp.  4.  Lay  a  five  shilling  piece  on  a  larger  plate  of  zinc ;  on  the  coin 
place  a  live  leech  or  a  live  snail ;  so  long  as  the  creature  does  not  come 
into  contact  with  the  zinc,  he  appears  perfectly  at  his  ease ;  but  the 
moment  he  moves  so  as  to  touch  the  zinc,  thereby  completing  the  con- 
nection between  the  two  metals,  he  receives  a  shock  and  instantly 
recoils. 

Exp.  5.  Place  a  silver  spoon  S  in  a  glass  containing 
a  solution  of  sulphate  of  copper ;  into  the  same  glass 
insert  a  strip  of  zinc  Z.  No  change  takes  place  in 
either  of  the  metals,  so  long  as  they  are  apart :  bring 
the  upper  ends  of  the  metals  in  contact  with  each  other ; 
the  silver  spoon  will  become  coated  with  copper,  which 
will  adhere  so  firmly  that  mere  friction  will  not  take 
it  off. 

4.  This  experiment  fully  illustrates  the  electrotype         Fig.  40. 
process. 

Exp.  6.   Place  a  slip  of  copper  C  (see  Fig.  41)  in  a  glass  containing 


VOLTAIC    ELECTRICITY. 


307 


Fig.  41. 


hydrochloric  acid  ;  into  the  same  glass  insert  a  strip  of  zinc  Z ;  so  long 
as  the  metals  remain  apart,  no  chemical  action  can  be  seen,  and  no  elec- 
tricity is  developed ;  bring  the  upper  extremi- 
ties d  of  the  metals  into  contact ;  active  decom- 
position immediately  begins  ;  the  chlorine  com- 
bines with  the  zinc,  and  the  hydrogen,  set  free, 
makes  its  appearance  at  the  surface  of  the  cop- 
per in  the  form  of  minute  bubbles  —  voltaic 
electricity  is  in  action.  "Withdraw  the  ex- 
tremities of  the  metals  from  each  other ;  the 
electrical  circuit  is  broken  —  electrical  action 
no  longer  exists;  restore  the  contact,  and  the 
electrical  action  is  again  renewed. 

The  disengagement  of  electricity  is  always  in 
proportion  to  the  chemical  action ;  and  the  metal 
which  is  most  acted  upon  by  the  fluid  gives 
off  its  negative  fluid  to  the  other  plate,  and  the 

consequence  of  this  is,  that  the  current  proceeds  from  this  latter  plate  to 
the  former.  In  the  experiment  just  given,  the  zinc  plate  is  acted  upon 
by  the  acid,  and  the  voltaic  current  proceeds  from  the  upper  extremity 
of  the  copper  plate  to  the  zinc  plate,  as  shown  in  the  figure. 

The  cheapest  acid  for  generating  small  portions  of  voltaic  electricity 
with  zinc  and  copper  plates  is  sulphuric  acid,  diluted  with  about  twelve 
parts  of  water  to  one  of  the  strong  acid. 

Exp.  7.  Bend  the  zinc  Z  as  shown  in  Fig.  42  ;  place 
a  bit  of  blotting  paper,  moistened  with  iodide  of  potas- 
sium, upon  the  zinc  at  A ;  bring  the  extremity  B  of  the 
strip  of  copper  C  (or  platinum)  in  contact  with  the 
moistened  paper ;  the  current  of  the  electric  fluid  passes 
in  the  direction  of  the  arrow  ;  the  iodide  of  potassium 
is  decomposed  by  the  electric  current ;  the  iodine  is 
evolved  at  the  positive  pole,  and  the  alkali,  free  potassa, 
at  the  negative  pole. 


Fig.  42. 


The  experiment  will  be  more  striking  if  a  drop  of  a  solution  of  starch 
be  also  added  to  the  moistened  paper. 

Exp.  8.  Perform  the  same  experiment  with  the  bibulous  paper  moist- 
ened with  prussiate  of  potassa,  slightly  acidulated  with  hydrochloric 
acid. 

Exp.  9.  Twist  a  piece  of  copper  wire,  in  the  form  of  a  spiral  or  helix, 
round,  a  small  glass  tube  ;  connect  the  extremities  of  the  wire  with  the 
zinc  and  copper  plates  immersed  in  diluted  sulphuric  acid ;  insert  a  steel 
needle  into  the  glass  tube ;  after  a  short  time,  the  needle  will  be  found 
to  be  powerfully  magnetic. 


308 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Exp.  10.  Bend  a  piece  of  soft  iron  wire  H  into  the  form  of  a  horse- 
shoe magnet ;  twist  a  piece  of  copper  wire,  covered  with  silk  or  cotton, 
round  this  wire,  as  shown  in  Fig.  43  ;  connect  the  extremities  of  this 


Fig.  43. 


Fig.  44. 


wire  with  the  voltaic  plates  Z  and  C ;  the  horseshoe  wire  H  instantly 
becomes  a  magnet.  If  you  have  no  covered  wire  at  hand,  wrap  a  piece 
of  paper  round  the  horseshoe  wire  before  you  twist  the  copper  wire 
round  it. 

Exp.  11.  Place  the  copper  wire  C  Z,  connecting  the  voltaic  plates,  in 
the  plane  of  the  magnetic  meridian ;  bring  a  magnetic  needle  exactly 
over  the  wire  C  Z ;  the  needle  will  be  deflected  from  its  north  and  south 
direction  by  the  action  of  the  wire  C  Z,  conducting  the  voltaic  current : 
now  bring  the  needle  exactly  below  the  wire  C  Z ;  the  needle  will  be 
deflected  to  the  side  opposite  to  that  towards  which  it  was  before  de- 
flected. 


BATTERIES  AND  THE  DIRECTION    OF  THE  VOLTAIC  CURRENT. 

5.  Fig.  45  represents  a  single  pair  of  zinc  and  copper  plates  acted 
upon  by  a  diluted  acid  ;  the  connecting  wire  C  Z  shows  the 
direction  of  the  electric  fluid ;  that  portion  of  the  copper 
plate  which  is  immersed  in  the  acid  becomes  charged  with 
negative  electricity,  and,  as  a  necessary  result  of  the  law  of 
electrical  repulsion,  the  positive  fluid  is  driven  off  from  the 
upper  extremity ;  hence  the  direction  of  the  current.  In  all 
batteries,  the  current  will  always  proceed  from  the  wire  at- 
tached to  the  metal  least  acted  upon  by  the  decomposing 
fluid. 

In  Volta's  battery,  represented  in  Fig.  39,  of  which  all 


Fig.  45. 


VOLTAIC    ELECTRICITY.  OUU 

other  batteries  may  be  regarded  as  mere  modifications,  the  wire  P  at- 
tached to  the  last  copper  plate  will  be  a  positive  or  -f-  pole,  and  the  wire 
Q  attached  to  the  first  zinc  plate  will  be  a  negative  or  —  pole.. 

Fig.  46  represents  a  voltaic  arrangement  of  two  plates  Pt.,  Pt.,  of  the 
same  metal,  viz.,  platinum,  immersed  in  different  fluids,  —  A  an  alkali, 
and  c  concentra'ted  nitric  acid,  separated  by  a  porous  partition  a  b.  The 
platinum  immersed  in  the  alkali  becomes  positively  electrified,  and  that 
in  the  acid  negatively  electrified,  and  the  current  flows  as  shown  in  the 
figure. 


PI-     + 


Fig.  47  shows  the  voltaic  action  which  takes  place  when  different 
metals  are  immersed  in  different  fluids.  The  platinum  plate  Pt.  is  im- 
mersed in  concentrated  nitric  acid  M,  and  the  zinc  plate  Z  in  concen- 
trated sulphuric  acid  S,  the  two  acids  being  separated  from  each  other 
by  the  porous  partition  a  b.  In  this  case,  the  most  intense  electromotive 
tension  is  excited,  the  one  metal  being  immersed  in  that  fluid  which 
renders  it  most  powerfully  negative,  and  the  other  metal  in  that  fluid 
which  renders  it  most  powerfully  positive.  This  is  the  principle  upon 
which  Groove's  battery  acts,  which  is  certainly  the  most  powerful  that 
has  yet  been  constructed. 


DIFFERENT   FORMS    OF    THE    VOLTAIC    BATTERY. 

6.  Cruickshank 's  battery,  represented  in  Fig.  48,  consists  of  an  oblong 
trough  of  baked  wood,  divided  into 
cells,  to  be  filled  with  acid,  by  a 
number  of  pairs  of  rectangular  plates 
of  zinc  and  copper.  This  form  was 
a  decided  improvement  on  the  com- 
mon pile,  but  still  it  had  several  in- 
conveniences in  practice. 

The    arrangement    represented  in 


310 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  49  removed  many  of  these  inconveniences.  It  is  merely  a  slight 
modification  of  the  Couronne  de  Tasses,  represented  in  Fig.  39.  The 
trough  T,  with  its  cells,  is  made  of  wedgewood  ware ;  the  plates  of  zinc 
and  copper,  forming  each  pair,  are  soldered  together  by  a  connecting 


Fig.  49. 

rod  at  the  top,  leaving  them  sufficiently  apart  at  the  bottom  to  span 
the  partitions  of  the  trough.  The  plates,  thus  joined  in  pairs,  are  all 
attached  to  a  wooden  bar  C  D,  by  which  they  may  be  readily  let  down 
into  the  trough,  when  they  are  required  to  be  in  action,  or  withdrawn 
from  the  trough  when  the  action  is  to  be  suspended. 

The  greatest  advantage  attending  this  arrangement  is,  that  by  simply 
raising  the  plates,  the  fluid  may  remain  in  the  trough  while  the  action 
of  the  battery  is  suspended. 


Fig.  50. 

Wollaston's  battery.  —  Wollaston,  having  discovered  that  the  effect  of 
the  foregoing  battery  was  augmented  by  increasing  the  surface  given  to 


VOLTAIC    ELECTRICITY. 


311 


the  copper,  he  enveloped  the  zinc  plate  of  each  pair  with  the  copper  plate 
of  the  preceding  one,  taking  care,  at  the  same  time,  to  avoid  all  metallic 
contact  between  these  two  plates.  By  this  arrangement,  the  copper 
plate  has  double  the  surface  of  the  zinc  plate. 

Fig.  51  represents  a  more  convenient  form  of  this  battery,  where  the 
trough  is  replaced  by  a  series  of  glass  jars.     The  acid  can  be  more  easily 


Fig.  51. 

changed  or  discharged  from  these  separate  cells  than  from  the  cells  in 
the  trough  of  the  preceding  form  of  the  apparatus. 


Fig.  52. 


Fig.  53. 


The  helical  battery.  —  In  this  battery,  the  zinc  and  copper  are  wound 
into  the  form  of  a  helix,  and  plunged  into  a  glass  vessel  containing  the 


312 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


diluted  acid  ;  the  helix  of  the  zinc,  in  each  pah-,  must  not  be  in  metallic 
contact  with  that  of  the  copper ;  but  the  helix  of  zinc  in  one  vessel  must 
be  in  metallic  communication  with  the  helix  of  copper  of  the  succeed- 
ing pair,  and  so  on,  as  in  Wollaston's  battery.  Fig.  52  shows  the  man- 
ner of  forming  each  pair  of  helixes,  and  Fig.  53  the  trough  adaptation 
for  each  pair.  This  arrangement  presents  a  great  surface  of  metal  in 
each  pair  to  the  action  of  the  acid,  without  the  necessity  of  having  very 
large  troughs.  The  acid  mixture  for  charging  this  battery  is,  water 
mixed  with  one  fortieth  in  volume  of  strong  sulphuric  acid  and  one  six- 
tieth of  nitric  acid. 

The  batteries  hitherto  described  all  have  one  decided  inconvenience, 
viz.,  that  after  a  short  time  they  lose  their  power,  which  occasions  them 
to  act  with  a  variable  force  during  the  same  course  of  experiments.  The 
zinc  plates  soon  become  covered  with  sulphate  of  zinc,  and  the  copper 
plates  with  hydrogen  and  even  oxide  of  zinc ;  these  deposits  not  only 
greatly  reduce  the  power  of  the  battery  when  in  use,  but  also  require 
the  plates  to  be  cleaned  before  being  put  into  action  again.  In  order  to 
avoid  these  inconveniences,  Daniell,  Grove,  and  Bunsen  invented  their 
constant  batteries. 


CONSTANT   BATTERIES. 

7.  These  batteries  are  constructed  on  the  principle  explained  in  con- 
nection with  Fig.  47,  p.  309  ;  where  the  porous  partition,  without  inter- 
rupting the  conduction  of  the  voltaic  fluid,  prevents  the  accumulation 
of  depositions  upon  the  plates. 

Daniell' s  constant  battery.  —  A  single  pair  of  this 
battery  is  represented  in  Fig.  54.  A  is  a  copper  ves-  — 
sel ;  C  a  porous  cell,  into  which  is  inserted  a  cylinder 
of  amalgamated  zinc  B  ;  a  and  b  are  binding  screws 
for  connecting  the  respective  metals  with  others  in 
the  series,  or  for  attaching  conducting  wires  when  a 
single  pair  only  is  to  be  used.  The  cell  C  into  which 
the  zinc  dips  is  filled  with  diluted  sulphuric  acid, 
(one  of  strong  acid  to  about  twenty  of  water ;)  and 
the  copper  vessel  A  is  filled  with  a  saturated  solution 
of  sulphate  of  copper.  So  long  as  the  poles  a  and  b 
are  disconnected,  no  action  will  take  place ;  but  the 
moment  the  circuit  is  completed  by  connecting  the 
screws  a  and  b,  a  very  powerful  action  occurs ;  the  pjgt  54.. 

inner  surface  of  the  copper  vessel  becomes  gradually 
covered  with  a  layer  of  pure  copper,  and  the  porous  cell  C  becomes 


VOLTAIC    ELECTRICITY.  ol<J 

charged  with  sulphate  of  zinc.  This  process  tenda  to  deprive  the  solu- 
tion of  sulphate  of  copper  of  its  copper,  and  to  neutralize  the  sulphuric 
acid  by  the  dissolution  of  the  zinc ;  in  order,  therefore,  to  sustain  the 
action  unimpaired,  some  crystals  of  sulphate  of  copper  are  placed  upon 
a  perforated  shelf  c,  in  contact  with  the  solution. 

Fig.  55  shows  the  manner  in  which  a  series  of  these  cells  are  con- 
nected so  as  to  form  a  battery. 


Fig.  55. 

The  advantages  of  this  battery  are  as  follows  :  (1.)  The  solution  of 
the  zinc  is  kept  apart  from  the  copper  by  the  porous  cell.  (2.)  The  hy- 
drogen, instead  of  escaping,  as  in  the  ordinary  batteries,  combines  with 
the  oxygen  of  the  oxide  of  copper,  and  precipitates  pure  copper  upon  the 
side  of  the  copper  vessel,  and  does  not  in  the  slightest  degree  affect  the 
action  of  the  battery.  (3.)  There  are  no  noxious  fumes  arising  from  the 
action  of  the  battery.  (4.)  The  amalgamation  of  the  zinc,  without  at 
all  affecting  the  production  of  electricity  by  the  battery,  prevents  the  zinc 
from  being  dissolved  by  the  sulphuric  acid  when  the  battery  is  not  in 
use  —  that  is  to  say,  when  its  poles  are  not  united  by  a  conductor  ;  but 
the  moment  this  union  takes  place,  the  zinc  is  attacked  by  the  acid  just 
as  if  the  mercury  were  not  there,  only  the  oxide  that  is  formed  does  not 
adhere  to  the  surface  of  the  metal,  which  it  would  do  if  the  zinc  were 
not  amalgamated,  and  would  thereby  tend  to  weaken  the  action  of  the 
battery.  Plates  of  zinc  are  very  easily  amalgamated  by  pouring  mer- 
27 


314         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


cury  upon  the  zinc,  and  at  the  same  -time  rubbing  it  on  the  surface 
•with  a  piece  of  chamois  leather  dipped  in  diluted  sulphuric  acid,  which, 
cleans  the  surface  of  the  zinc,  and  thereby  brings  the  mercury  and  zinc 
into  combination. 

Grove  s  battery.  —  This  battery  is  con- 
structed on  the  same  principle  as  Dan- 
iell's ;  that  is  to  say,  the  plates  are  acted 
upon  by  two  liquids  separated  from  each 
other  by  a  porous  earthen  ware  partition.  A, 
The  pairs  of  plates  are  composed  of  amal- 
gamated zinc  Z  and  platinum  foil  Prf, 
plunged  into  a  cell  A  B  C  D  composed  of 
glazed  porcelain  or  glass.  The  cell  A  B  C  D 
is  filled  with  diluted  sulphuric  acid,  which 
acts  directly  iipon  the  zinc  ;  and  the  porous 
cell  a  into  which  the  platinum  is  plunged 
is  filled  with  nitric  acid.  The  platinum 
plate  Ptf  is  in  metallic  contact  with  the  zinc 
of  the  succeeding  cell,  as  shown  at  a ;  and 
so  on  to  the  whole  series  of  cells  in  the 


Fig.  56. 


battery.  As  the  power  of  these  batteries  is  much  increased  by  giving  to 
the  zinc  plates  a  very  large  surface  as  compared  with  the  surface  of  the 
platinum  plates,  the  zinc  plates  are  bent  round  the  porous  cell  a,  so  that 
they  form  in  each  cell  two  vertical  surfaces  united  by  a  horizontal  sur- 
face at  the  bottom. 

When  the  poles  of  this  battery  are  united,  so  as  to  bring  it  into  ac- 
tion, the  hydrogen  arising  from  the  decomposition  of  diluted  acid  does 
not  attach  itself  to  the  platinum,  but  goes  to  change  the  nitric  acid 
into  nitrous  acid ;  the  oxide  of  zinc  remains,  as  in  Daniell's  battery,  in 
the  cell  of  the  diluted  acid,  without  penetrating  through  the  porous  cell 
to  the  platinum,  which  consequently  remains  perfectly  clean ;  this  cir- 
cumstance essentially  contributes  to  keep  up  the  power  and  constancy  of 
the  battery,  which  render  it  so  valuable  as  a  voltaic  combination.  After 
a  time,  however,  the  nitrous  acid,  which  is  constantly  formed,  acquires  a 
high  temperature,  and  gives  off  deleterious  fumes ;  when  this  takes  place, 
the  action  of  the  battery  should  be  arrested.  This  battery,  for  almost 
every  purpose,  is  the  most  powerful  that  has  yet  been  constructed.  About 
half  a  dozen  cells,  with  a  platinum  surface  in  each  of  ten  square  inches, 
will  be  found  sufficiently  powerful  for  performing  all  the  most  interest- 
ing experiments  connected  with  voltaic  electricity. 

Bunsen's  battery  differs  from  Grove's  only  in  charcoal  being  substituted 
for  the  platinum.  The  cells  of  this  battery  have  the  cylindrical  form, 


VOLTAIC    ELECTRICITY. 


315 


represented  in  Fig.  57,  in  consequence  of  the  carbon  or  charcoal  being 
best  made  in  the  form  of  hollow  cylinders.    The  strip  of  copper  A  B 


Fig.  57. 

shows  how  the  zinc  of  one  cell  is  united  with  the  carbon  cylinder  of  the 
succeeding  cell ;  and  so  on.  The  figure  also  shows  how  the  strips  of 
copper  C  and  D  forming  the  poles  of  the  battery  are  connected  with  the 
extreme  cells  of  the  battery. 

Each  charcoal  cylinder  usually  carries  a  collar  of  copper  at  its  upper 
end  for  the  purpose  of  fixing  the  connecting  strip  of  copper  to  it ;  this 


Fig.  58. 


316         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


copper  stands  above  the  glass  vessel  so  as  not  to  come  in  contact  with  the 
nitric  acid ;  however,  it  is  found  that,  owing  to  the  porous  nature  of  the 
charcoal,  the  acid,  to  a  certain  extent,  rises  to  the  copper  collar,  and  in 
time  destroys  its  efficiency.  The  following  contrivance  (see  Fig.  58) 
completely  remedies  this  inconvenience :  the  charcoal  cylinder  C  is  solid  ; 
and  into  its  top  is  thrust  a  stout  copper  rod  N  M,  which  is  bent  so  as  to 
be  brought  into  contact  with  the  succeeding  zinc  cell.  To  prevent  the 
acid  ascending  to  this  copper  rod,  the  top  of  the  charcoal  cylinder  sur- 
rounding the  wire  is  covered  with  a  cement  of  wax. 

It  is  almost  unnecessary  to  say  that  the  charcoal  cylinder  in  this  bat- 
tery is  plunged  into  the  nitric  acid,  filling  the  porous  tube  or  cell,  and 
that  this  porous  cell  is  placed  within  the  cylinder  of  amalgamated  zinc, 
which  is  surrounded  by  the  diluted  sulphuric  acid  filling  the  glass  jar 
or  porcelain  vessel. 

Smee's  battery.  —  In  this  battery,  the  plates  are  acted  upon  by  only 
one  liquid,  viz.,  diluted  sulphuric  acid,  (one  part  of  acid  to  about  seven 
parts  of  water.  Fig.  69  represents  one  of  the  cells  of 
this  battery.  A  the  earthen  ware  cell  filled  with  the 
diluted  acid;  Z  Z  two  vertical  plates  of  amalga 
mated  zinc,  one  on  each  side  of  the  platinized  plate  of 
silver  S  ;  w  a  bar  of  wood,  to  which  these  plates  are 
fixed ;  b  a  binding  screw,  which  secures  the  zinc  plates 
to  the  wooden  bar.  The  connections  are  made,  as 
usual,  by  means  of  the  small  binding  screws  shown 
in  the  figure.  This  forms  a  highly  economical  and 
efficient  battery.  Although  its  power  may  be  less  than 
Grove's  battery,  at  the  same  time  it  is  in  many  re- 
spects more  convenient  and  agreeable  for  general  use, 
especially  for  conducting  experiments  relating  to  elec- 
tro-magnetism. Fig.  59. 


VOLTAIC    ELECTRICITY. 


317 


VOLTAMETERS. 

8.  These  instruments  are  used  for  measuring  the  power 
of  a  battery.  There  are  three  kinds  of  voltameters  at  pres- 
ent in  use ;  one  of  them  depends  upon  the  decomposing  power 
of  the  battery,  another  upon  its  heating  power,  and  the  third 
upon  its  magnetizing  power. 

It  has  already  been  shown  how  the  poles  of  a  battery  resolve  water 
into  its  constituent  gases  —  hydrogen  and  oxygen.  Now,  it  is  presumed 
that  this  decomposing  power  of  a  battery  is  in  proportion  to  its  general 
power,  or  rather  in  proportion  to  the  quantity  of  electric  fluid  developed 
by  the  battery.  Now,  in  the  gas  voltameters  represented  in  Figs.  60,  61, 
and  62,  the  quantity  of  gas  liberated  by  the  poles  of  the  battery  in  a 
given  time  is  taken  as  the  measure  of  the  power  of  the  battery ;  or,  what 
amounts  to  the  same  thing,  the  power  may  be  measured  according  to 
the  inverse  ratio  of  the  time  requisite  for  liberating  a  given  volume  of 
the  gas. 

In  Fig.  60,  the  platinum  poles  of  the  battery  are  placed  vertically  in 
a  graduated  glass  tube  D,  very  near  to  each  other ;  the  lower  extremities 


Fig.  60. 


Fig.  61. 


A  and  B  come  out  at  the  bottom  vessel  V  containing  the  water,  so  that 
a  connection  may  be  readily  made  between  them  and  the  wires  leading 

27* 


318 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


from  the  extreme  plates  of  the  battery.  The 
two  gases  in  this  instrument  are  received  in  the 
same  tube ;  they  may,  however,  be  received  in 
separate  tubes,  as  in  the  instrument  represented 
in  Fig.  37  ;  but  in  this  case,  the  platinum  poles, 
being  at  a  greater  distance  from  each  other, 
cause  the  decomposition  to  go  on  more  slowly. 
When  the  battery  has  very  great  power,  the 
gases  are  usually  collected  in  a  graduated  re- 
ceiver D  (Fig.  61)  placed  upon  the  pneumatic 
trough  N  ;  the  decomposition  of  the  water  takes 
place  in  a  bottle  M  fitted  up  with  a  cork  and 
bent  tube  A  for  conveying  the  gases  to  the  re- 
ceiver D ;  the  platinum  poles  or  electrodes  P, 
hanging  near  together  within  the  bottle,  are  con- 
nected with  the  wires  B  and  C  leading  to  the 
extreme  plates  of  the  battery. 

Fig.  62  represents  an  eligible  form  of  this  ap- 
paratus, when  the  liberation  of  gas  is  very  fee- 
ble. The  quantity  of  gas  is  measured  by  the 
amount  of  displacement  of  the  liquid.  A  grad- 
uated tube  A  proceeds  laterally  from  the  lower 
part  of  the  vessel  V  wherein  the  decomposition 
of  the  water  is  carried  on  ;  so  that,  as  the  gases 
rise  to  the  top  of  the  closed  vessel  V,  an  equal 
volume  of  water  is  thrown  up  the  tube  A. 

The  three  following  voltameters  depend  upon  the  calorific  effects  of  the 
battery. 

O 


Fig.  62. 


Fig.  63. 


Fig.  64. 


VOLTAIC    ELECTRICITY.  319 

Fig.  63  represents  a  voltameter,  which  is  merely  a  slight  modification 
of  the  common  pyrometer.  The  platinum  wire  a  b  forming  a  portion 
of  the  voltaic  circuit  becomes  powerfully  heated,  and  expands,  and 
allows  the  pointer  a  d  to  fall ;  the  graduation  on  the  quadrant  indicates 
the  amount  of  expansion,  and  consequently  the  relative  power  of  the 
battery. 

Fig.  64  represents  a  still  more  sensible  voltameter,  in  which  the  plati- 
num wire  forming  a  portion  of  the  voltaic  circuit  passes  through  the  ball 
of  an  air  thermometer ;  the  expansion  of  the  air  by  the  heat  of  the  wire 
causes  the  liquid  to  rise  in  the  vertical  graduated  tube ;  and  so  on. 

The  magnetic  voltameter  will  be  hereafter  fully  described. 


EFFECTS   OF  VOLTAIC  ELECTRICITY. 

CHEMICAL    EFFECTS. 

9.  The  chemical  action  of  voltaic  electricity  upon  different 
substances  placed  in  the  circuit  forms  one  of  its  most  impor- 
tant and  remarkable  features.  It  has  been  shown  that  frictional 
electricity  exerts  a  chemical  action ;  but  it  is  very  feeble  as 
compared  w^th  that  which  even  small  voltaic  batteries  exert. 

One  of  the  most  remarkable  facts  connected  with  these 
decompositions  is,  that  certain  elements,  into  which  the  sub- 
stances are  resolved,  always  arrange  themselves  on  the  pos- 
itive pole  or  electrode,  and  certain  other  elements  always 
arrange  themselves  on  the  negative  pole  or  electrode.  Thus 
oxygen,  chlorine,  iodine,  and  the  acids  always  fly  to  the  pos- 
itive pole  of  the  battery ;  and  hydrogen,  alkalies,  oxides,  &c., 
always  fly  to  the  negative  pole.  For  example,  in  the  decom- 
position of  water,  (see  page  304,)  the  oxygen  is  accumulated 
in  the  tube  placed  over  the  positive  pole,  while  the  hydrogen 
is  accumulated  in  that  placed  over  the  negative  pole.  This 
fact  has  led  to  the  formation  of  a  system  of  electro-chemistry. 
The  respective  poles  are  supposed  to  be  in  a  contrary  electri- 
cal state  to  the  elements  which  they  attract ;  hence  oxygen, 
chlorine,  acids,  &c.,  are  negative  elements,  and  alkalies,  ox- 
ides, &c.,  are  positive.  Every  chemical  compound,  therefore, 


320 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


consists  of  a  negative  element  and  a  positive  element,  which 
are  held  united  by  the  law  of  electrical  attraction. 

Exp.  1.  Decomposition  of  icater.  —  This  is  most  successfully  performed 
by  the  apparatus  described  at  page  304  ;  but  the  following  simple  form 
of  making  the  experiment  is  highly  instructive. 

Immerse  a  strip  of  amalgamated  zinc  and  a  strip  of  clean  copper  into 
a  glass  of  water  slightly  acidulated  with  sulphuric  acid  :  so  long  as  the 
metals  are  kept  apart,  no  action  can  be  observed ;  but  the  instant  that 
the  upper  extremities  of  the  metals  are  brought  into  contact,  the  water  is 
decomposed,  numerous  little  bubbles  of  hydrogen  collect  round  the  cop- 
per, and  the  oxygen  at  the  same  time  passes  to  the  zinc,  and  oxidizes  it. 

Exp.  2.  Decomposition  of  neutral  salts.  —  Fill  the  two  tubes  of  the 
apparatus  represented  in  Fig.  37,  page  304,  with  a  solution  of  sulphate 
of  soda,  or  any  other  neutral  salt,  colored  blue  with  tincture  of  violets ; 
then,  when  the  battery  is  in  action,  the  acid  will  be  attracted  to  the  pos- 
itive electrode,  and  will  render  the  liquid  in  the  tube  red,  and  the  alkali 
will  be  attracted  to  the  negative  electrode,  and  will  tinge  the  liquid  in 
the  tube  green ;  transpose  the  poles,  and  the  effects  will  be  reversed. 

Exp.  3.    To  precipitate  a  metal  from  the  solution  of  a  metallic  salt.  — 
Fig.  65  represents  a  simple  piece  of  apparatus  for  producing 
this  decomposition,     a  is  a  glass  tube  about  an  inch  in  diam- 
eter, having  a  piece  of  bladder  tied  over  its  lower  extremity, 
and  suspended  in  a  large  glass  vessel  b  ;  pour  a  solution*  of 
acetate  of  lead  (sulphate  of  copper,  nitrate  of  silver,  &c.,  mil 
answer  the  purpose)  into  the  glass  tube  a ;  fill  the  outer  vessel 
with  diluted  sulphuric  acid ;  into  the  solution  of  lead  im- 
merse a  platinum  wire  p,  and  into  the  diluted  sulphuric  acid 
plunge  a  strip  of  amalgamated  zinc  Z,  in  metallic  contact 
with  the  platinum ;  then  a  voltaic  circuit  will  be  formed,  con- 
sisting of  two  metals  and  two  exciting  fluids ;  the  acetate  of  lead  will 
be  decomposed,  the  metallic  lead  will  be  attracted  to  the  platinum,  and 
the  acid  to  the  zinc. 

The  metallic  vegetations  called  the  lead  tree,  &c.,  depend  upon  the 
voltaic  action. 

Exp.  4.  Connect  the  tin  foil  plate  G  with 
the  copper  pole  of  a  small  battery,  and  the 
metal  plate  D  with  the  zinc  pole  of  the 
battery.  Lay  a  piece  of  white  blotting  pa- 
per, soaked  in  a  diluted  solution  of  hydro- 
chloric acid  and  prussiate  of  potassa,  upon 
the  plate  D  ;  draw  a  number  of  strokes 
with  a  brush  dipped  in  varnish  across  the 
plate  G,  as  shown  in  Fig.  66 ;  take  a  bent  wire  A,  and  connect  the  two 


Fig.  65. 


Fig.  66. 


VOLTAIC    ELECTRICITY.  321 

plates  with  it ;  move  the  wire  from  end  to  end  ;  then  the  electro  circuit 
will  be  complete  whenever  the  wire  connects  the  metallic  foil  and  the 
damp  paper,  and  the  circuit  will  be  broken  at  those  parts  where  the  wire 
passes  over  the  varnish ;  the  solution  on  the  paper  will  be  decomposed  at 
the  former  parts,  but  will  remain  unchanged  at  the  latter  parts,  as  will 
be  shown  by  the  deep-blue  marks  formed  upon  the  paper  by  the  decom- 
position of  the  prussiate  of  potash. 

This  experiment  illustrates  the  principle  on  which  the  copying  elec- 
tric telegraph  is  constructed. 

.Electrotyping . 

10.  The  decomposition  and  reduction  of  metals,  in  a  state 
of  solution,  by  the  voltaic  battery,  has  led  to  some  important 
discoveries  in  the  arts  —  such  as  electrotyping,  electroplat- 
ing, &c. 

The  experiment  given  in  connection  with  Fig.  49,  page  310,  is  a 
familiar  example  of  electroplating;  electrotyping  depends  upon  the 
same  principle. 

Fig.  67  represents  a  very  simple  contrivance  for  ob- 
taining small  electrotype  casts.  A  is  a  glass  vessel,-  in 
which  the  mould  from  which  the  cast  is  to  be  obtained 
is  placed.  B  is  a  glass  tube  suspended  within  the  ves- 
sel A  by  means  of  a  metallic  ring  d  e,  to  which  are 
fixed  three  strips  of  metals.  The  lower  end  of  this 
tube  m  n  is  closed  by  tying  a  piece  of  bladder  over  it. 
The  strip  of  brass  b  c  has  two  binding  screws  b  and  a, 
by  which  the  wires  a  Z  and  b  k  are  secured ;  the  low- 
er extremities  of  these  wires  carry  the  substances  Z 
and  k,  which  act  as  the  electromotors  or  voltaic  plates 
constituting  the  simple  battery.  Z  is  an  amalgamated  zinc  plate,  sus- 
pended within  the  tube ;  k  is  the  body  from  which  the  cast  is  to  be 
taken,  laid  on  the  spiral-shaped  extremity  of  the  wire  b  k  ;  the  model 
is  the  negative  electromotor,  and  Z  the  positive  electromotor.  The  large 
glass  A  contains  a  concentrated  solution  of  sulphate  of  copper ;  this  is 
always  kept  in  a  saturated  state  by  having  crystals  of  sulphate  of  cop- 
per suspended  in  it ;  the  tube  B  is  filled  with  diluted  sulphuric  acid ;  the 
liquids  should  stand  at  the  same  level  in  both  vessels.  According  to  this 
arrangement,  the  electric  current  passes  from  the  zinc  Z  to  the  mould 
k ;  and  pure  metallic  copper  is  deposited  upon  the  surface  of  the  mould, 
and  in  time  forms  a  complete  cast  of  the  surface. 

The  following  particulars  should  be  attended  to  in  making  electrotype 


322 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


casts :  The  surfaces  of  the  originals  should  be  good  conductors,  and  they 
should  not  contain  any  substances  which  would  be  acted  upon  by  the 
sulphate  of  copper.  The  model  may  be  of  gypsum,  wax,  or  any  similar 
non-conducting  substance,  provided  the  surface  of  the  model  be  covered 
with  a  metallic  coating  ;  plumbago  dust  or  bronze  powder,  laid  on  with 
a  camel's  hair  brush,  forms  a  very  good  coating.  A  wax  impression 
should  be  first  slightly  moistened  with  alcohol,  and  then  the  black  lead 
should  be  rubbed  over  it  with  a  stiff  brush ;  the  copper  wire  should  then 
be  warmed  and  pressed  into  the  margin  of  the  wax ;  the  connection  be- 
tween the  wax  and  wire  should  then  be  made  with  the  black  lead.  A 
coating  of  wax  dissolved  in  turpentine  will  protect  any  part  of  a  coin 
from  any  metallic  deposit.  In  every  electrotype  cast,  the  elevated  por- 
tions of  the  original  will  be  depressed,  and  vice  versa ;  in  order,  therefore, 
to  obtain  exact  fac-similes  of  the  original,  the  first  cast  must  be  used  as 
a  matrix,  on  which  the  coating  of  copper  must  be  thrown  by  the  elec- 
trotype process. 

Fig.  68  represents  a  more  convenient  arrangement. 
z  is  the  amalgamated  zinc  rod,  suspended  in  the  po- 
rous cell  p,  which  contains  the  diluted  sulphuric  acid ; 
c  the  glass  or  earthen  ware  vessel  containing  the  solu- 
tion of  sulphate  of  copper ;  to  the  copper  wire  con- 
nected with  the  zinc  by  a  binding  screw,  and  carrying 
at  its  lower  extremity  the  seal  or  mould  m, 

Pig.  69  represents  another  arrangement,  where  the 
current  is  generated  in  a  distinct  battery  A ;  B  is  a 
distinct  vessel  for  conducting  the  electrotype  process, 
m  is  a  metal  rod  for  supporting  the  moulds,  and  c 
another  rod  supporting  copper  plates,  which  form  the 
positive  electromotors ;  x  connects  the  moulds  with  the 
zinc  in  the  battery,  and  z  connects  the  copper  plates  with  the  copper 


Fig.  69. 


VOLTAIC    ELECTRICITY.  323 

plates  of  the  battery.  The  vessel  B  contains  two  parts  of  saturated 
solution  of  sulphate  of  copper  and  one  part  of  sulphuric  acid  diluted 
with  about  seven  parts  of  water.  The  action  which  takes  place  is  sim- 
ply as  follows :  the  copper  is  consumed  from  the  plates  c,  and  deposited 
on  the  moulds  m  ;  so  that  the  copper  solution  in  the  vessel  B  remains 
unchanged  in  its  strength  from  the  commencement  to  the  close  of  the 
process. 

ELECTROPLATING    EXPERIMENTS. 

11.  Exp.  1.  Place  a  small  plate  of  clean  platinum  foil  in  a  saucer,  and 
pour  over  it  a  solution  of  sulphate  of  copper,  covering  it  to  the  depth 
of  about  a  sixteenth  of  an  inch  ;  touch  the  platinum  plate  with  a  point- 
ed strip  of  bent  zinc,  the  other  end  of  which  is  kept  in  the  liquid  ;  dif- 
ferent colored  rings  of  metal  will  be  formed  upon  the  platinum  plate. 

Exp.  2.  The  brilliancy  of  the  depositions  will  be  much  increased  by 
using  a  constant  battery  of  three  or  four  pairs  of  cells. 


PROTECTION    OP   METAL    PLATES    BY   VOLTAIC    CURRENTS. 

12.  Voltaic  currents  not  only  affect  combinations  and  de- 
compositions, but  they  may  also  be  employed  for  impeding  or 
arresting  certain  decompositions  which  would  otherwise  take 
place  by  the  ordinary  laws  of  chemical  affinities.  Thus,  for 
example,  Davy  protected  the  copper  bottoms  of  ships  from 
the  corrosion  of  the  salt  water  by  connecting  plates  of  zinc 
with  the  copper  sheathing.  In  order  to  protect  a  metal  from 
the  chemical  action  of  an  acid  or  a  saline  solution,  it  is  only 
necessary  to  combine  the  metal  with  some  other  metal  in  the 
fluid,  which  shall  act  as  the  positive  electromotor. 

Exp.  1.  Place  a  copper  plate  in  a  saucer ;  pour  some  diluted  sulphuric 
acid  upon  it :  then  the  metal  will  be  violently  acted  upon  by  the  acid ; 
now  touch  the  copper  with  a  strip  of  zinc,  and  the  action  on  the  copper 
will  be  instantly  arrested ;  the  action  will  now  be  transferred  to  the  zinc, 
and  the  copper  will  remain  unaffected  by  the  acid,  until  the  whole  of  the 
zinc  be  dissolved. 


324          NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

LUMINOUS    AND    HEATING   EFFECTS    OF   VOLTAIC 
ELECTRICITY. 

13.  The  most  brilliant  of  all  the  effects  of  voltaic  electricity  is  the 
arch  of  electrical  light  evolved  between  two  charcoal  points.  This  phe- 
nomenon may  be  exhibited  with  about  a  dozen  pairs  of  Grove's  or  Bun- 
sen's  battery.  This  experiment  may  be  most  conveniently  performed  by 


Fig.  70. 

fixing  charcoal  points  (pieces  of  graphite  answer  best)  to  the  rods  of  a 
universal  discharger.  The  poles  of  the  battery  are  resjlectively  connected 
with  the  extremities  of  the  rods,  as  shown  in  Fig.  70,  where  the  arch 
A  B  represents  the  form  of  the  voltaic  light.  The  points  must  be  first 
brought  into  contact  with  each  other,  and  then  gradually  withdrawn 
until  the  arch  attains  its  most  brilliant  appearance ;  the  length  of  the 
arch  of  course  varies  with  the  power  of  the  battery ;  that  is,  with  bat- 
teries of  average  power,  from  three  quarters  of  an  inch  to  about  an  inch, 
and  a  half.  This  arch  of  flame  is  not  produced  by  combustion,  for  it 
may  be  exhibited  with  equal  brilliancy  in  a  vacuum,  and  even  takes 
place  under  water. 

The  intense  heat  of  this  electric  arch  will  ignite  the  most  refractory 
substances. 

Exp.  1.  Amalgamate  the  ends  of  the  polar  wires;  bring  them  near 
together,  while  the  battery  is  in  action ;  a  white  starlike  spark  will  be 
seen,  accompanied  with  a  crackling  noise  similar  to  the  emission  of  a 
feeble  spark  of  frictional  electricity. 

Exp.  2.  The  spark  obtained  from  these  amalgamated  points  may  be 
seen  under  water,  or  in  the  flame  of  a  candle. 

Exp.  3.  Immerse  one  of  the  wires  into  mercury,  and  bring  the  end 
of  the  other  wire  near  the  surface  of  the  fluid ;  a  spark  is  emitted  just 
before  the  wire  touches  the  mercury,  leaving  a  small  black  speck  upon 
its  surface. 

Exp.  4.  Coat  the  ends  of  the  polar  wires  with  soot,  by  holding  them 
for  a  short  time  in  the  flame  of  an  oil  lamp ;  the  sparks  obtained  from 
these  coated  wires  will  be  much  stronger  and  brighter. 

The  power  of  a  voltaic  battery,  as  we  have  already  shown, 
is  roughly  estimated  by  the  heat  which  it  produces  in  a  given 
conducting  wire.  The  temperature  to  which  a  conducting 


VOLTAIC    ELECTRICITY.  325 

wire  will  be  raised  by  a  battery,  depends  upon  the  length  and 
thickness  of  the  wire,  as  well  as  upon  the  nature  of  the  metal, 
whether  or  not  it  is  a  good  or  a  bad  conductor  of  electricity. 
Short  fine  wires  become  most  heated,  and  of  all  metallic  wires, 
platinum,  being  the  worst  conductor  of  electricity,  becomes 
most  powerfully  heated  by  conducting  the  electric  fluid. 

The  calorific  effect  appears  to  depend  more  upon  the  size 
of  the  plates  than  upon  the  number  of  pairs  ;  that  is  to  say, 
it  depends  upon  the  quantity  of  the  electric  fluid  evolved, 
rather  than  upon  its  intensity  or  tension. 

The  calorific  effects  of  the  voltaic  battery  may  be  most 
conveniently  shown,  by  stretching  the  wires  to  be  heated  be- 
tween the  extremities  of  the  rods  of  the  universal  discharger. 
(See  Fig.  70.) 

Exp.  1.  To  show  the  heating  power  of  a  battery.  Stretch  a  piece  of 
fine  steel  wire  between  the  poles  of  the  battery ;  the  wire,  if  it  is  not  too 
long,  will  instantly  become  powerfully  incandescent.  If,  on  the  first 
trial,  the  wire  only  presents  a  dull  heat,  gradually  reduce  the  length  of 
the  wire,  until  it  glows  with  a  white  heat ;  reduce  the  length  of  the 
wire  a  little  more ;  then  it  will  be  first  fused,  and  then  ignited. 

The  same  experiment  may  be  performed  with  platinum  or  silver  wire. 

Exp.  2.  Ether,  alcohol,  phosphorus,  gunpowder,  &c.,  may  be  readily 
ignited  by  making  the  hot  platinum  connecting  wire  to  pass  through 
them,  or  to  touch  some  portion  of  them. 

Exp.  3.  If  the  platinum  wire  be  conducted  through  a  small  portion 
of  water,  it  will  speedily  boil. 

,        PHYSIOLOGICAL    EFFECTS    OF   VOLTAIC    ELECTRICITY. 

14.  The  relation  between  voltaic  action  and  the  nervous 
system  of  animals  was  very  carefully  investigated  at  a  very 
early  stage  of  the  history  of  voltaic  electricity. 

The  peculiar  nature  of  this  relation  is  explained  at  page 
303,  when  a  small  battery  is  employed.  But  with  large  bat- 
teries the  effects  are  truly  surprising.  Dr.  Ure  thus  de- 
scribes his  experiments  upon  the  body  of  a  full-grown  man, 
fifteen  minutes  after  death.  Upon  applying  one  of  the  polar 
wires  to  the  forehead,  and  the  other  to  the  heel,  every  muscle 
28 


326          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

in  his  countenance  was  simultaneously  thrown  into  fearful 
action ;  rage,  horror,  despair,  anguish,  and  ghastly  smiles 
united  their  hideous  expression  in  the  murderer's  face.  At 
this  period,  several  of  the  spectators  were  forced  to  leave  the 
apartment  from  terror  and  sickness,  and  one  gentleman 
fainted. 

The  physiological  effects  of  voltaic  electricity  appear  to 
depend  upon  intensity,  rather  than  upon  quantity  ;  that  is  to 
say,  upon  the  number  of  pairs  in  the  battery,  rather  than 
upon  their  extentgof  surface. 

The  effect  of  the  voltaic  shock  is  much  increased  by  at- 
taching copper  cylinders  to  the  extremities  of  the  conducting 
wires,  and  also  by  dipping  the  hands,  by  which  the  shock  is 
received,  in  water  slightly  acidulated. 

The  magnetic  effects  of  voltaic  electricity  are  so  various 
and  interesting,  that  they  have  been  treated  as  a  distinct 
branch  of  electrical  science,  called  Electro-Dynamics. 


ELECTRO-DYNAMICS. 


ELECTRO-MAGNETISM. 

1.  IT  has  already  been  shown  (Exp.  9,  p.  307)  how  a  steel 
needle  may  be  magnetized  by  passing  a  voltaic  current 
through  a  wire  helix.  When  a  helix  is  wound  to  the  right,  or 
in  the  direction  of  a  corkscrew,  it  is  called  a  right-handed 
helix,  as  shown  in  Fig.  71,  and  when  the  helix  is  wound  in 


Fig.  71.  Fig.  72. 

the  contrary  direction,  that  is,  to  the  left,  it  is  called  a  left- 
handed  helix,  as  shown  in  Fig.  72.  Helix  wires  should  be 
formed  of  copper  wire,  covered  with  silk,  for  the  purpose  of 
insulating  them. 

When  a  needle  is  magnetized  by  a  current  passing  through 
a  right-handed  helix,  or  a  corkscrew  helix,  the  south  pole  of 
the  needle  is  always  at  the  extremity  through  which  the  cur- 
rents enter  ;  that  is  to  say,  at  the  extremity  that  is  in  connec- 
tion with  the  positive  electricity.  On  the  contrary,  when  a 
.  needle  is  magnetized  by  a  left-handed  helix,  the  north  pole  is 
at  the  extremity  which  is  in  connection  with  the  positive 
electricity. 

These  facts  are  in  exact  accordance  with  Ampere's  theory  of  magnet- 
ism, (see  p.  301 ;)  for  the  electric  current  moves  round  the  magnetic 
bar  in  precisely  the  same  manner  as  the  magnetic  current  is  supposed  to 
do  in  that  theory,  thereby  showing  that  the  electric  ciirrent  which  in- 
duces the  magnetic  condition  is  equivalent  to  the  magnetic  current  upon 
which  the  ordinary  magnetic  condition  is  supposed  to  depend. 

Let  S  N  (Fig.  73)  be  a  right-handed  helix,  that  is,  a  corkscrew  helix, 

(327) 


323 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


through  which  the  electric  current  enters  at  S,  and  passes  out  at  N ; 
then,  from  what  has  been  said,  the  helix  will  become  a  magnet,  having 
the  extremity  S  for  its  south  pole,  and  N  for  its  north  pole.  This  may 
be  tested  experimentally  by  using  Do  la  Rive's  floating  lattery.  The 
extremities  of  the  helix  are  connected  with  zinc  and 
copper  plates  Z  and  C,  fixed  in  a  piece  of  cork,  so  as 
to  make  the  whole  apparatus  to  float  in  a  strongly 
acidulated  liquid.  This  float  battery,  like  the  floating 
needle,  will  place  itself  in  the  north  and  south  direc- 
tion of  the  needle ;  the  extremity  S,  through  which 
the  current  enters,  will  be  directed  to  the  south. 

The  author  has  found  the  following  form  of  this  apparatus  to  be  very 
convenient  in  practice. 

B  is  a  deal  board,  having  two 
concentric  grooves  E  and  F  cut  in 
it,  and  filled  with  mercury ;  the 
wires  N  and  M  connect  the  mer- 
cury in  these  grooves  with  the 
binding  screws  C  and  Z,  to  which 
the  poles  of  the  battery  are  at- 
tached ;  S  N  is  a  corkscrew  helix 


Fig.  74. 


surrounding  a  soft  bar  of  iron ;  one  extremity  of  the  wire  dips  into  the 
mercury  of  the  groove  F,  and  the  other  into  that  of  the  groove  E ;  the 
soft  iron  bar,  with  its  helix,  turns  upon  the  pivot  A.  When  the  positive 
pole  of  the  battery  is  fixed  to  the  binding  screw  C,  and  the  negative 
pole  to  the  binding  screw  Z,  the  helix,  with  its  soft  iron  bar,  becomes  an 
actual  magnetic  needle,  and  will  settle  itself  in  the  direction  of  the 
magnetic  meridian,  the  extremity  S  being  directed  to  the  south,  and  the 
other  extremity  N  to  the  north. 

If  the  soft  iron  bar  be  taken  away,  and 
a  steel  needle  be  inserted  in  its  place,  the 
needle  will  be  magnetized,  having  the  ex- 
tremity towards  S  a  south  pole,  and  the 
extremity  towards  N  a  north  pole. 

Fig.  75  represents  another  form  of  the 
floating  battery,  where  the  copper  and 
zinc  plates  are  immersed  in  a  glass  tube, 
filled  with  the  diluted  sulphuric  acid,  and 
the  whole  is  made  to  float  in  a  vessel  of  . 
water. 

Electro-magnets  of  immense  power  may 
be  formed  by  voltaic  helices. 


ELECTRO-DYNAMICS.  329 


The  Electro-Magnet,  or  soft  iron  Horseshoe  Magnet. 

2.   Fig.  76  represents  an  electro-magnet ;  M  is  the  soft  iron  bent  in 
the  form  of  a  horseshoe  magnet ;  P  and  N  are  the  extremities  of  the 
helix  of  covered  copper  wire,  surrounding 
the  bar  in  the  manner  just  described ;  K  is  M 

the  keeper  of  the  magnet,  from  which  a 
heavy  weight  may  be  suspended,  to  show 
the  power  of  the  magnet.  When  the  ex- 
tremities, P  and  N,  are  connected  with 
the  poles  of  a  single  pair  of  any  of  our 
constant  batteries,  the  soft  iron  instantly 
becomes  a  very  powerful  magnet,  capable 
of  supporting  a  weight  of  1  cwt.  to  about 
1  ton.  The  moment  the  connection  is 
broken,  the  magnet  loses  its  power.  The 
wire  intended  to  form  the  coil  is  cut  into 
several  portions,  and  is  coiled  separately  pigf  75. 

on  the  iron,  and  then  all  the  corresponding- 
extremities  are  collected  into  parcels,  which  are  soldered  to  a  thick  wire, 
which  communicates  with  the  pole  of  the  battery.  By  this  arrange- 
ment, the  current  is  divided  into  a  series  of  short  branches,  which,  col- 
lectively, communicate  with  the  poles  of  the  battery  by  a  short,  strong 
wire ;  this  gives  energy  to  all  the  coils,  and  thereby  increases  the  power 
of  the  electro-magnet. 

These  temporary  magnets  have  been  called  electro-magnets, 
to  distinguish  them  from  permanent  steel  magnets,  and  electric 
helices  just  described. 


Rotating  Magnets. 

3.  The  rotating  magnet,  invented  by  Dr.  Richie,  is  represented  in  Fig. 
77.  In  this  instrument,  a  permanent  rotatory  motion  is  given  to  an  electro- 
magnet c,  upon  a  vertical  pivot,  by  means  of  the  alternate  attraction  and 
repulsion  of  the  poles  N  and  S,  of  a  permanent  horseshoe  magnet.  In 
order  to  produce  this  continuous  rotation,  it  is  requisite  that  the  poles  of 
the  electro-magnet  should  be  reversed  at  every  time  they  pass  the  poles 
of  the  permanent  magnet ;  this  is  effected  by  a  very  simple  and  elegant 
artifice :  a  b  is  a  wooden  cup  of  mercury,  divided  into  two  compart- 
ments by  a  bridge  or  partition  of  wood,  in  a  line  with  the  poles  N  and 
S,  whose  upper  edge  is  a  little  below  the  exterior  edge  of  the  cup ;  so 
28* 


330 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


that  when  the  two  compartments  are  filled  with  mercury,  the  cohesion 
of  the  particles  of  the  fluid  causes  it  to  stand  a  little  higher  than  the 
level  of  the  top  edge  of  the  partition ;  the  two  extremities  of  the  helix 
dip  a  little  into  the  mercury  without  reaching  to  the 
level  of  the  top  of  the  partition,  so  that  the  electro- 
magnet may  freely  revolve  upon  its  vertical  pivot ;  the 
mercury  in  one  of  the  compartments  is  connected  with 
the  positive  pole  of  a  small  battery,  and  the  other  com- 
partment with  the  negative  pole  ;  by  this  contrivance, 
the  poles  of  the  electro-magnet  are  reversed  at  every 
time  the  dipping  wires  cross  the  partition,  and,  conse- 
quently, if  the  poles  of  the  permanent  magnet  attract 
the  poles  of  the  electro-magnet  in  any  given  position, 
they  will  be  repelled  the  moment  the  dipping  wires 
have  crossed  over  the  partition,  and  thus  the  contin- 
uous rotation  is  sustained. 

The  following  is  a  brief  description  of  a  rotatory 
magnet  invented  by  the  author  some  twenty  years  ago,  and  employed 
by  him  in  an  extended  form  as  a  magnetic  engine,  capable  of  yielding 
about  a  quarter  of  a  horse  power. 
N  S  is  the  electro-magnet,  turning 
upon  a  horizontal  axis  A  B ;  N  F 
and  S  E  are  the  terminal  wires  of 
the  coil;  each  of  them  forks  off 
into  two  branches,  F  H,  F  J,  and 
E  K,  and  E  G  ;  the  extremities 
of  the  wires  are  connected  with 
metal  segments,  H,  J,  K,  and  G, 
attached  to  the  ivory  wheels  A  and 
B,  fixed  to  the  common  axis  A  B  ; 
the  circumferences  of  these  seg- 
ments  are  placed  concentric  with 
the  axis  of  motion,  and  their  edges  dip  into  mercury  placed  in  the  cups 
L  and  M,  which  are  connected  with  the  poles  of  the  battery :  by  this 
contrivance  the  poles  of  the  electro-magnet  are  changed  when  one  pair 
of  segments  passes  out  of  contact,  and  another  pair  comes  into  contact, 
with  the  mercury  in  the  cups.  The  opposite  poles  of  a  permanent  mag- 
net are  placed  in  a  line  with  the  electro-magnet  when  its  position  cor- 
responds with  the  change  of  its  polarity,  as  in  the  case  of  Richie's 
rotating  magnet. 


ELECTRO-DYNAMICS.  331 

Contact  Breakers.  —  Telegraphic  Alarm   Bell. 

4.  The  two  foregoing  pieces  of  apparatus  show  how  the  poles  of  an 
electro-magnet  may  be  reversed  by  changing  the  direction  of  the  voltaic 
current.  The  contrivance  represented  in  Fig.  79,  called  a  contact 
breaker,  shows  with  what  rapidity  an  electro-magnet  can  acquire  and 
lose  its  magnetism.  M  is  a  small  electro-magnet,  the  armature  of 
which,  E,  is  capable  of  oscillating  between  the  two  poles  of  the  magnet 
and  a  stop  at  its  back,  against  which  it  is  pressed  by  a  spring.  The 
conducting  wire  D  coils  round  the  lower  branch  of  the  magnet, 
as  shown  in  the  figure,  and  the  other  conducting  wire  C  is  at- 
tached to  the  stop,  and  then  a  wire  passes  from  the  foot  of  the  oscil- 


Fig.  79. 

lating  armature  to  the  extremity  of  the  coil  passing  round  the  upper 
branch  M  of  the  electro-magnet ;  so  that  the  electric  current  is  com- 
plete when  the  armature  is  in  contact  with  the  stop,  and  it  is  broken 
when  this  contact  is  destroyed.  The  consequence  of  this  arrangement 
is,  the  electro-magnet  attracts  the  armature,  which  breaks  the  circuit, 
and  the  magnetism  instantly  ceases ;  then  the  armature,  being  pressed 
back  by  the  spring,  returns  and  strikes  the  stop,  which  again  completes 
the  circuit  and  renews  the  magnetism  ;  the  armature  is  again  attracted 
by  the  magnet,  and  so  on  with  great  rapidity.  The  adjusting  screws 
F  and  G  enable  the  operator  to  regulate  the  rapidity  of  the  strokes. 

To  form  this  instrument  into  an  alarm  bell,  it  is  only  requisite  to  fix 
a  hammer  to  the  top  of  the  armature  E,  and  to  place  a  bell  within  the 
striking  distance. 


332  NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

Instruments  for  measuring  the  Force  of  Magnets. 

5.   Method  of  contact.  —  The  following  is  a  simple  contrivance  for 
estimating  the  suspensive  force  of  an  electro-magnet :  N  J  S  is  the  elec- 


Fig.  80. 

tro-magnet ;  p  and  m  the  binding  screws  to  which  the  poles  of  the  bat- 
tery are  fixed ;  A  the  feeder  or  armature,  suspended  from  the  extremity 
a  of  a  graduated  lever  a  b  turning  on  a  fixed  centre  or  fulcrum  c ;  h  a 
sliding  hook,  to  which  a  scale  pan  with  weights  may  be  attached.  The 
weights,  put  in  the  scale  pan,  necessary  for  breaking  the  contact  of  the 
feeder  A,  give  the  data  for  calculating  the  force  of  the  magnet,  on  the 
simple  principle  of  the  lever  of  the  first  kind. 

Method  of  vibrations.  —  The  oscillations  of  a  magnetic  needle,  before 
it  settles  in  its  north  and  south  direction,  follow  the  same  law  as  the 
vibrations  of  the  pendulum.  The  directive  force  of  a  magnetic  needle, 
therefore,  may  be  measured  by  the  number  of  oscillations  that  it  will 
make  in  a  given  time  when  drawn  a  little  to  one  side  of  its  magnetic 
meridian.  When  the  same  needle  is  employed  to  determine  the  directive 
force  at  two  different  places  on  the  earth,  this  directive  force  varies  as 
the  squares  of  the  number  of  vibrations  performed  in  a  given  time. 

The  vibration  of  the  needle  is  also  employed  to  determine  the  intensity 
of  the  different  points  in  a  magnetic  bar.  In  this  case,  it  is  necessary 
that  an  allowance  should  be  made  for  the  directive  force  of  the  earth. 

According  to  the  experiments  made  by  Coulomb,  the  at- 
tractive forces  of  the  different  points  in  a  long  magnetic  bar, 
as  estimated  from  the  centre  of  the  bar,  increase  in  a  geomet- 
rical progression  as  the  distances  from  the  centre  increase  in 
arithmetical  progression. 


ELECTRO-DYNAMICS. 


333 


TO   MAGNETIZE    STEEL    BARS    BY   THE   ELECTRO-MAGNETIC 
COIL. 


6.  The  simplest  way  of  doing 
this  is  to  coil  a  very  stout  copper 
wire,  covered  with  silk,  round  a 
pasteboard  tube,  about  18  inches 
long  and  2  inches  diameter.  The 
bar  A  B  to  be  magnetized  is  placed 
between  two  soft  iron  cores,  A  Q 
and  B  D,  made  exactly  to  fit  the 
interior  of  the  pasteboard  tube 
E  F.  The  whole  is  placed  within 
the  tube,  and  the  extremities  C  and 
Z  of  the  helix  are  connected  with 
the  poles  of  the  battery  :  in  a  short 
time  the  steel  bar  A  B  will  be 
magnetized  to  saturation. 


ON  THE  ACTION  OF  ELECTRIC  AND  MAGNETIC 
CURRENTS. 

7.  In  addition  to  the  magnetic  effects  of  electrical  currents, 
which  have  just  been  noticed,  the  following  general  laws  of 
electro-dynamics  have  been  established :  — 

General  Laws  of  Electro-Dynamic  Action. 

a.  Every  metallic  conductor,  through  which  an  electric 
current  passes,  acts  on  magnets  suspended  freely,  and  shows 
magnetic  properties. 

5.  Electric  currents  exert  on  each  other  influences  like 
those  which  they  exert  on  magnets. 

c.  A  magnet  acts  on  an  electric  current  precisely  as  an- 
other current  would  do. 

d.  Electric  currents  in  conductors  in  like  manner  excite 
such  currents. 

e.  Magnets  can  in  like  manner  excite  electric  currents  and 
the  other  electric  influences  dependent  on  them. 


334 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


It  must  be  observed,  that  the  condition  essential  to  these 
effects  is,  that  the  electric  fluid  must  be  in  a  state  of  motion, 
,  that  is,  it  must  be  in  the  form  of  a  continuous  current ;  or, 
in  other  words,  it  must  be  in  the  condition  which  is  called 
dynamic.  There  is  no  action  when  the  electricity  is .  in  the 
static  or  tension  state. 


ACTION    OF   ELECTRIC    CURRENTS    ON   THE   MAGNETIC 
NEEDLE. 

8.  Oersted's  experiment.  —  Place  the  conducting  wire  A  B  of  a  bat- 
tery in  the  direction  of  the  magnetic  meridian,  viz.,  B  towards  the  north 
and  A  towards  the  south,  as  shown  in  Fig.  82  ;  suspend  a  needle  S  N 


over  the  conducting  wire  A  B,  and  the  north  pole  N  will  be  deflected  to 
the  east ;  suspend  the  needle  below  the  conducting  wire  A  B,  and  its 
north  pole  will  be  deflected  to  the  west. 

The  needle,  therefore,  endeavors  to  assume  a  position  perpendicular  to 
the  direction  in  which  the  electric  current  flows. 

Ampere  represented  the  action  of  the  electric  current  on  a  magnetic 
needle  under  a  form  which  is  easily  remembered.  "  "We  have  only  to 
conceive  a  man  lying  down  in  the  portion  of  the  circuit  under  consider- 
ation, in  such  a  manner  that  the  current  enters  by  his  feet,  and  goes  out, 
consequently,  by  his  head ;  furthermore,  we  have  but  to  conceive  that 
this  man  has  always  his  face  turned  towards  the  needle,  so  as  to  look  at 
it ;  then  the  action  is  always  found  to  be  such  that  the  north  pole  of  the 


ELECTRO-DYNAMICS.  335 

needle  is  deviated  to  the  left  of  this  man.  This  formula  comprehends 
all  possible  cases." 

It  is  easy  to  see  that  the  positive  current,  coming  from  the  positive 
pole  of  the  battery,  passes  along  the  conductor,  and  arrives  at  the  nega- 
tive pole,  and  returns  through  the  plates  of  the  battery  to  the  positive 
pole ;  so  that  the  current  has  a  different  direction  in  the  two  parallel 
portions  of  the  circuit,  as  sh(*vn  in  Fig.  82. 

All  these  effects  are  perfectly  in  accordance  with  the  theory  of  mag- 
netic action  explained  at  page  301.  The  needle  seeks  to  place  itself  at 
right  angles  to  the  direction  of  the  current,  on  the  principle  that  the 
electric  current  in  the  magnet  seeks  to  place  itself  parallel  to  the  current 
in  the  wire. 

Galvanometers. 

9.  We  have  explained  the  construction  of  certain  voltameters  or 
galvanometers  depending  upon  the  calorific  and  chemical  effects  of  the 
voltaic  current ;  but  the  most  perfect  instrument  of  this  kind  is  that 
which  depends  upon  the  magnetic  effects  of  the  current. 

The  most  simple  magnetic      P      N 
galvanometer  is  represented  in     <Tj>  Q  ^ 

Fig.  83.    A  magnetic  needle       y       j__ fa 

n  s  is  suspended  between  two       '  -  \          •-  u 

conducting    parallel    wires  w 

and  W,   terminating  in  the  „.     go 

mercury  cups  P  and  N.    The 

conducting  wire  is  placed  in  the  direction  of  the  magnetic  meridian,  so 

that  the  needle  has  the  same  direction  as  the  wires.     When  the  poles 

of  the  battery  are  inserted  in  the  cups  P  and  N,  the  needle  is  deflected 

after  the  manner  described  in  the  foregoing  section.     According  to  this 

arrangement,  the  conducting  wire  above  the  needle,  as  well  as  the  wire 

below  it,  tends  to  deflect  the  needle  in  the  same  direction,  so  that  the 

double  wires  exactly  double  the  amount  of  deflection.     The  angle  of 

deflection  gives  us  a  rough  mode  of  estimating  the  quantity  of  voltaic 

fluid  evolved  by  the  battery. 

But,  instead  of  bending  the  wire  round  the  needle  once,  if  we  bend  it 
twice,  thrice,  or  any  number  of  times,  we  must  obviously  increase  the 
deflecting  power  in  the  same  ratio. 
This  construction  is  adopted  in  the 
galvanometer  represented  in  Fig.  84 ; 
svhere  n  s  is  the  needle,  surrounded 
by  a  series  of  coils  of  covered  silk 
wire,  add.  This  instrument  has  been 
called  the  galvanometer  multiplier.  Fig.  84. 


!G 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  85  represents  a  more  elegant  form  of  the  instrument ;  where  the 
coil  of  wire  is  wound  round  a  wooden  frame  fixed  upon  a  stand,  and 
provided  with  binding  screws. 


Fig.  85. 


Fig.  86. 


Nobili's  galvanometer  multiplier,  represented  in  Fig.  86,  consists  of  an 
astatic  needle  (see  p.  300)  suspended  by  a  filament  of  untwisted  silk, 
one  of  the  needles  being  placed  within  the  conducting  coil,  the  other 
without  it ;  so  that  the  current  of  electricity  tends  to  deflect  both  nee- 
dles in  the  same  direction,  thereby  giving  a  double  power  to  the  instru- 
ment. The  whole  of  the  coil,  together  with  the  needle  and  its  thread 
of  suspension,  is  covered  with  a  glass  shade ;  a  and  b,  fixed  to  the  bind- 
ing screws,  are  the  wires  proceeding  from  the  poles  of  the  current,  whose 
power  is  to  be  determined  by  the  instrument ;  the  extremities  of  the 
wire  coil,  of  course,  terminate  in  these  binding  screws. 


ACTION  OF  ELECTRIC  CURRENTS  ON  EACH  OTHER. 

10.  Ampere  discovered  the  following  laws,  according  to 
which  electric  currents  act  upon  each  other :  — 

a.  Parallel  currents  attract  each  other  when  they  flow  in 
the  same  direction. 


ELECTRO-DYNAMICS. 


337 


Thus  the  parallel  wires  a  b  and  c  d,     a 
represented  in    Fig.   87,   transmitting 
currents  in  the  same  direction,  attract     c 
each  other. 


Fig.  87. 


b.  Parallel  currents  repel  each  other  when  they  flow  in 
contrary  directions. 


Fig.  88. 


Thus  the  parallel  wires  a  b  and  c  d,    a~~ 

represented  in  Fig.   88,   transmitting     c 

contrary  currents,  repel  each  other. 

These  laws  are  perfectly  in  accord- 
ance with  the  theory  of  magnetism  explained  at  p.  301. 

In  order  to  establish  these  laws  by  experiment,  the  floating  battery 
represented  in  Fig.  89  may  be  employed.     This  battery  consists  of  a  pair 


Fig.  89. 


of  plates,  viz.,  platinum  and  amalgamated  zinc,  fixed  to  a  cork  float  A, 
and  having  its  poles  in  connection  with  the  cups  a  and  b ;  the  wire 
frame  proceeding  from  these  cups  conducts  the  current  as  represented 
by  the  arrows  in  the  figure  ;  the  whole  of  this  floating  battery  is  placed 
in  a  vessel  "containing  diluted  sulphuric  acid,  which  acts  as  the  exciting 
fluid.  To  one  of  the  vertical  branches  E  F  we  present  a  parallel  wire 
29 


338          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

C  D,  traversed  by  a  powerful  current  of  electricity ;  then  when  the  cur- 
rents flow  in  the  same  direction,  the  wire  E  F  with  its  floating  battery 
is  attracted  by  the  wire  C  D  ;  and,  on  the  contrary,  when  the  current 
in  the  wire  C  D  flows  in  a  contrary  direction,  the  floating  battery  is  re- 
pelled. 

The  same  laws  hold  true  with  res*pect  to  angular  currents, 
or  those  currents  whose  directions  are  inclined  to  each  other; 
the  form  of  expression  in  this  case  being  simply,  that  currents 
which  are  directed  to  the  same  point,  or  which  proceed  from 
the  same  point,  attract  each  other,  and  vice  versa,  as  before. 


VARIOUS  MOTIONS  PRODUCED  BY  THE  MUTUAL  ACTION  OF 
MAGNETS  AND  CURRENTS,  AND  CURRENTS  UPON  EACH 
OTHER. 

11.  The  oscillating  electrical  spiral,  represented  in  Fig.  90,  affords  a 
beautiful  illustration  of  the  attraction  of  parallel  currents. 
A  fine  flexible  copper  spiral  wire  A  is  suspended  from  the 
extremity  of  a  conductor  D,  proceeding  from  the  positive 
pole  of  the  battery ;  the  lower  extremity  of  this  spiral 
dips  slightly  into  a  cup  of  mercury,  a,  in  which  is  placed 
the  extremity  of  the  wire  C,  leading  from  the  negative 
pole  of  the  battery.  When  the  current  is  complete,  the 
spiral  vibrates  longitudinally ;  for  at  every  contraction  the 
current  is  broken,  and  then  the  weight  of  the  wire  causes 
its  extremity  to  sink  again  into  the  mercury,  and  thus  a  continuous 
oscillation  is  sustained. 

It  has  been  shown  that  a  fixed  or  closed  current  exerts  a  tangential 
force   upon   the  pole   of   a   magnet 
which  is  free    to  move :    thus  let 
A  B,  in  Fig.  91,  represent  the  di- 
rection of  the  fixed  current,  and  N       s 
the  pole  of  a  magnet,  free  to  obey 
the  impulse ;  then  the  north  pole  N         ^^       B"~™ 
is  impelled  in  the  tangential  direc-     s  j?/0.  91, 

tion  N  n  ;  that  is  to  say,  in  a  direc- 
tion perpendicular  to  the  direction  of  ,the  current  A  B. 

In  like  manner,  since  action  and  reaction  are  equal  and  contrary,  a 
pole  of  a  magnet  exerts  a  tangential  force  on  a  current  which  is  free 
to  move;  thus  the  pole  N  of  a  fixed  magnet  (see  Fig.  92)  will  impel  the 
free  wire  A  B  conducting  a  current  in  the  tangential  direction  B  a. 


ELECTRO-DYNAMICS. 


339 


These  results  are  generally  rep- 
resented in  Fig.  93,  where  N  rep-     S 
resents  the  north  pole  of  a  magnet, 
C  the  section  of  the  conductor  of 
a  descending  electric  current  per- 
pendicular to    the  plane  of  the 
paper ;  then  the  action  of  C  upon 
N  tends  to  move  it  in  the  direction  N  n,  and 
the  reaction  of  the  pole  N  upon  the  wire  C 
tends  to  move  it  in  the  contrary  direction  C  c. 
If,  therefore,  the  pole  N  be  free  to  move  round 
the  wire  C,  the  tangential  line  N  n  will  be 
the  direction  of  the  motion ;  and  if  the  con- 
ducting wire  C  be  free  to  move  round  the  pole 
N,  the  tangential  line  C  c  will  be  the  direc- 
tion of  the  motion. 

The  following  rotatory  motions  depend  on 
these  principles. 


a 

Fig.  92. 


c 

Fig.  93. 


To  make  the  pole    of  a   magnet    N   revolve   round  a  fixed 
electric  current  C. 

This  was  first  effected  by  Faraday  in  the  following  manner :     A  small 
magnet  N  is  fixed  to  the  lower  part  of  a  vessel  V,  by  means 
of  a  silk  thread ;  the  vessel  is  filled  with  mercury  nearly  to 
the  top  of  the  magnet ;  C  is  a  conducting  wire  dipping  into 
the  mercury,  and  Z  is  another  conductor  communicating 
with  the  mercury  at  the  bottom  of  the  vessel.    Now,  when 
the  electric  current  is  established,  by  connecting  the  ex- 
tremities of  the  wires  C  and  Z  with  the  opposite  poles  of 
the  battery,  the  pole  N  of  the  magnet  revolves  round  the 
conducting  wire  C.      The  ends  of  the  wires  should  be 
amalgamated  to  insure  metallic  contact.     If  the  current  is     Fig.  94 
descending,  that  is,  if  C  be  connected  with  the  positive 
pole  of  the  battery,  and  if  N  be  a  north  pole,  its  motion  round  the  wire 
will  be  direct,  that  is,  in  the  direction  of  the  hands  of  a  watch  ;  and  so 
on,  vice  versa. 


To   make    a   movable    wire    A  B,  traversed    by    a    current, 
revolve  round  the  pole  N  of  a  fixed  magnet. 

Here  the  wire  A  B  is  suspended  from  the  wire  C  by  a  loop,  and  dips 
into  the  mercury  in  the  vessel  V  ;  when  the  circuit  is  established,  by 


340 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


connecting  the  -wires  C  and  Z  with  the  respective  poles  of  the  battery, 
the  conducting  wire  revolves  round  the  pole  N  of  the  magnet. 

If  the  current  be  descending,  and  N  be  the  north  pole  of  the  magnet, 
the  rotation  will  be  direct. 

These  two  rotations  may  be  exhibited  in  one  piece  of  apparatus,  as 
represented  in  Fig.  96,  where  m  represents  the  revolving  small  magnet, 


Fig.  95. 

which  is  best  made  with  a  sewing  needle ;  fp  the  revolving  wire ;  c  the 
positive  pole  of  the  battery ;  and  z  the  negative  pole.  When  the  north 
poles  of  the  magnets  are  both  turned  upwards,  the  rotations  take  place 
in  the  directions  of  the  arrows,  as  shown  in  the  figure.  Reverse  the 
direction  of  the  electric  current,  and  the  rotations  will  be  reversed. 


Ampere's  rotation  of  a  current  about  the  pole  of  a  magnet. 

On  the  two  poles  N  and  S  of  a  permanent 
horseshoe  magnet  are  placed  two  cells  of  copper,  <* 
(a  c  c  e  n  on  N,  and  e  z  z  a  n  on  S ;)  b  d,  b  d,  c 
are  copper  wires  attached  to  cylinders  of  amal- 
gamated zinc,  which  dip  into  the  diluted  sul- 
phuric acid,  filling  the  cells ;  these  zinc  cylinders 
turn  on  pivots  at  s  s  ;  the  zinc  cylinders  revolve 
round  the  respective  poles  of  the  magnet  in  con- 
trary directions  ;  that  is,  in  the  directions  indi- 
cated by  the  arrows. 

Fig.  97. 


ELECTRO-DYNAMICS. 


341 


Fig.  98. 


Fig.  98  represents  a  slight  modification  of  the  foregoing ;  here  the 
copper  cell  turns  upon  a  pivot,  as  well  as  the  zinc  cylinder ;  and  for  an 
obvious  reason  they  revolve  in  contrary  directions. 


ELECTRO-DYNAMIC  INDUCTION. 

12.  Faraday  was  the  first  philosopher  who  discovered 
the  laws  of  electro-dynamic  induction.  He  showed  that  an 
electric  current,  or  a  magnet,  is  able  by  induction  to  develop 
at  a  distance  electric  currents  in  a  conducting  wire ;  in  the 
same  way  as  common  electricity  electrizes  an  insulated  con- 
ductor by  induction. 

Exp.  1.  To  show  the  induction  of  a  current  by  magnetism.  Take 
the  coil,  represented  in  Fig.  81,  and  place  its  extremities  C  and  Z  in 
connection  with  the  respective  binding  screws  of  a  galvanometer  ;  sud- 
denly insert  a  strong  cylindrical  magnet-  within  the  coil,  and  the  needle 
will  be  instantly  deflected,  but  it  will  almost  immediately  return  to  its 
original  position ;  suddenly  withdraw  the  magnet,  and  the  needle  will 
be  deflected  in  the  opposite  direction. 

Thus  it  appears  that  the  induction  of  the  current  acts  only 
at  the  instants  of  application  and  withdrawal  of  the  magnet. 

This  explains  the  principle  on  which  Clark's  magneto-electric  machine 
acts.  29  * 


3-12 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Exp.  2.  Attach  small  copper  cylinders  near  to  the  respective  extremi- 
ties of  the  wire  C  and  Z ;  and  place  a  bundle  of  soft  iron  rods  (insulated 
from  each  other  by  a  coating  of  shell  lac)  into  the  coil ;  connect  the 
wires  C  and  Z  with  the  poles  of  the  battery ;  hold  the  copper  cylinders 
in  the  hands,  and  suddenly  withdraw  one  of  the  wires  from  the  pole  of 
the  battery,  and  a  pretty  powerful  electric  shock  will  be  felt,  and  at  the 
same  time  a  spark  will  be  given  off  from  the  point  of  the  wire ;  at  the 
moment  of  restoring  the  contact,  another  shock  will  be  felt. 

The  current  produced  in  these  experiments  is  called  a  primary  current; 
a  secondary  current  is  produced  in  the  following  manner :  — 

Over  the  coil  of  wire  described  in  the  foregoing  experiments,  let  an 
exactly  similar  coil  be  formed  upon  it ;  let  Fig.  99 
represent  this  double  coil,  where  a  and  b  are  the 
ends  of  the  first  or  primary  coil,  c  and  d  the  ends 
of  the  second  or  secondary  coil.  Connect  the  ends 
a  and  b  with  the  poles  of  a  battery,  and  the  ends 
c  and  d  with  a  galvanometer  ;  then  the  needle  will 
be  instantly  deflected,  showing  that  a  secondary 
current  had  been  induced  in  the  second  coil  by  the 
primary  current  in  the  first  coil;  suddenly  take 
away  one  of  the  wires  from  the  cup  of  the  galva- 
nometer, and  the  needle  will  be  deflected  in  the 
opposite  direction.  The  induced  currents  only  exist  for  an  instant,  viz., 
at  the  instant  of  making  or  of  breaking  the  contact. 


MAGNETO-ELECTRIC    MACHINES. 

13.  One  of  the  most  simple  machines  of  this  kind  is  represented  in 
Fig.  100.  J  J  is  a  sectional  representation  of  a  double  induction  spiral; 
r  r  the  wooden  hollow  roller  on  which  the  primary  coil  of  stout  copper 
wire  a  a  is  wrapped  ;  b  b  the  secondary  coil 
of  fine  wire  surrounding  the  first  coil ;  m 
the  bundle  of  iron  wires  placed  in  the  hol- 
low axis  of  the  coils,  and  projecting  with 
its  lower  pole  a  little  beyond  £he  wooden 
cylinder  ;  one  end  z  of  the  wire  of  the  pri- 
mary coil  is  connected  with  one  pole  of  a 
constant  battery,  and  the  other  end  y  hfx 
of  the  wire  of  the  primary  coil  with  the 
other  pole  of  the  battery ;  that  portion  of 
the  conductor^  h  ef,  between  the  two  binding  screws  y  and  x,  acts  as 
the  contact  breaker.  This  contact  breaker  is  constructed  as  follows  :  it 


100. 


ELECTRO-DYNAMICS.  343 

is  soldered  at  f  to  a  flexible  plate  screwed  to  the  rod  proceeding  from 
the  binding  screw  x  ;  e  is  a  plate  of  soft  iron,  soldered  to  the  conducting 
wire,  exactly  under  the  electro-magnetic  rods  m  ;  at  h,  the  conducting 
wire  is  bent  downwards,  and  terminates  with  a  hammer,  having  a  plat- 
inum point,  which  rests  upon  a  copper  plate  or  anvil  p.  When  the 
hammer  h  is  in  contact  with  the  anvil  p,  the  electrical  current  is  com- 
plete, and  the  soft  iron  wires  m  become  powerfully  magnetized  by  the 
primary  current ;  the  magnet  then  attracts  the  plate  e,  and  breaks  the 
contact,  the  rods  instantly  lose  their  magnetism,  and  then  the  hammer 
h  falls  upon  the  anvil  mt  and  thereby  again  restores  the  electrical  cur- 
rent ;  and  so  on.  This  process  goes  on  with  great  rapidity,  so  long  as 
the  connection  of  the  wires  z  and  y  with  the  poles  of  the  battery  is 
maintained. 

Vivid  sparks  are  emitted  between  the  hammer  and  the  anvil,  every 
time  the  connection  is  broken  or  made. 

Substances,  to  be  subject  to  the  action  of  the  electric  current,  must  be 
interposed  between  the  binding  screws  x  and  y  ;  thus  the  thermal,  chem- 
ical, magnetizing,  and  physiological  effects  may  be  observed  at  the 
instant  the  contact  of  the  hammer  with  the  anvil  is  broken  or  destroyed. 

But  the  secondary  current  is  that  which  should  be  used  for  pro- 
ducing the  shocks  or  physiological  effects.  For  this  purpose,  the  ex- 
tremities of  the  wire  forming  the  secondary  coil  b  b  are  soldered  to  small 
copper  cylinders,  and  these  are  held  in  the  hands  of  the  person  wishing 
to  receive  the  shocks,  one  cylinder  in  each  hand.  A  rapid  succession  of 
shocks  is  felt,  for  the  effect  takes  place  at  every  time  the  contact  of  the 
hammer  with  the  anvil  is  broken  or  renewed. 

This  machine  has  been  constructed  in  various  forms ;  sometimes 
Richie's  rotating  magnet  is  used  for  breaking  and  renewing  the  connec- 
tion of  the  conducting  wire  of  the  primary  coil  with  the  poles  of  the 
battery. 

Faraday's  Magneto-electric  Machine. 

14.  The  first  machine  of  this  kind  was  constructed  by  Faraday,  as 
shown  in  Fig.  101.  It  is  thus  described  by  Brand  in  his  Manual  of 
Chemistry. 


344 


NATUKAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  101. 

C  is  a  copper  plate,  so  mounted  as  to  admit  of  revolving  on  its  axis  ; 
n  s  are  the  poles  of  a  powerful  horseshoe  magnet,  so  placed  as  to  admit 
of  the  revolution  of  the  plate  between  them ;  w  w'  are  conducting  wires, 
one  of  which  is  retained  in  metallic  contact  with  the  axis,  and  the  other 
with  the  rim  of  the  plate,  at  the  point  between  the  poles  n  s.  These 
wires  are  connected  with  the  galvanometer  g.  When  the  copper  plate  is 
made  to  revolve  from  right  to  left,  a  current  of  electricity  is  produced  in 
the  direction  of  the  arrows,  and  deflects  the  galvanometer  accordingly. 


Clark's  Magneto-electric  Machine. 

15.  Pixii  first  made  a  machine  of  this  kind,  which  was  successively 
improved  by  Saxton  and  Clark.  The  arrangement  adopted  by  Clark  is 
thus  described  by  M.  Becquerel  in  his  treatise  on  Electricity. 

A  (Fig.  102)  represents  a  series  of  six  magnetized  bars  of  steel,  bent 
into  a  horseshoe  form,  arranged  vertically,  and  supported  by  four  screws 
fixed  to  the  board  B,  two  of  which  are  seen  at  M  N,  (Fig.  103.)  A 
thick  bar  of  brass  C  is  pierced  in  its  centre  by  an  opening,  into  which 
passes  a  bolt  with  a  nut  for  the  purpose  of  securing  the  magnet  against 
the  board  B.  By  this  arrangement  the  magnet  may  be  easily  removed 
without  disturbing  the  rest  of  the  apparatus.  D  represents  the  arma- 
ture of  a  double  cylinder  of  soft  iron  G  F,  which  is  fixed  to  a  brass 
screw  placed  between  the  poles  of  the  battery  A.  This  piece  is  set  in 
motion  in  the  manner  indicated  in  Fig.  103,  by  means  of  the  wheel  E 
of  an  axis  of  rotation  and  an  endless  cord.  On  each  cylinder  is  rolled 
a  helix  of  fine  copper  wire,  coated  with  silk,  and  about  800  yards  in 
length.  One  of  the  ends  of  each  helix  is  soldered  to  the  armature ; 


ELECTRO-DYNAMICS. 


345 


perpendicular  to  the  surface  of  which,  at  D,  is  a  brass  rod  supporting 
two  break-pieces  H.  K  represents  a  hollow  brass  cylinder,  to  which  is 
soldered  one  of  the  free  ends  of  the  helices,  and  which  is  separated  from 


Fig.  102. 


the  rod  by  means  of  a  piece  of  hard  wood  resting  on  it ;  the  other  end 
of  the  helices  is  in  communication  with  the  rod.  O  is  an  iron  wire 
spring  to  exercise  a  pressure  against  the  hollow  cylinder  K,  with  which 
it  is  in  metallic  contact,  by  means  of  a  screw  fixed  in  the  brass  plate  M. 
P  represents  a  square,  vertical  brass  rod,  fitted  into  the  brass  plate  N. 
Q  is  a  metal  spring,  exercising  a  feeble  pressure  on  the  break  piece  H : 
it  is  held  in  metallic  contact  by  means  of  a  binding  screw.  T  is  a  cop- 
per wire  for  making  communication  between  the  brass  plates  M  N.  By 
means  of  this  arrangement,  these  various  parts  D,  H,  Q,  P,  N,  are  in 
connection  with  one  of  the  ends,  and  K  and  M  with  the  two  other  ends. 
It  is  very  evident  that,  as  the  spring  Q  presses  gently  on  the  break-piece 
H,  the  effects  are  regular It  is  very  necessary  that  the  break- 


346 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


piece  be  so  arranged  that  the  spring  Q  shall  separate  at  the  very  time 
when  the  iron  cylinders  of  the  armature  are  leaving  the  poles  of  the 
magnet.  With  respect  to  the  iron  wire  O,  it  always  exercises  a  gentle 


Fig.  103. 

pressure  against  the  hollow  brass  cylinder  K.  By  means  of  these  ar- 
rangements, a  mercury  bath,  which  is  always  inconvenient,  is  super- 
seded. When  the  shock  is  to  be  given  by  this  machine,  the  two  copper 
conductors  R  S  (Fig.  102)  are  taken  into  the  hands,  which  are  moistened 
with  salt  w.ater,  one  of  the  conductors  being  in  communication  with  the 
plate  M,  and  the  other  with  the  plate  N,  in  the  manner  shown  in  the 
figure ;  M  and  N  are  then  united  by  the  piece  T.  The  shock  received 
by  this  apparatus  as  soon  as  the  wheel  is  turned  is  very  violent.  If  we 
desire  a  current  always  in  the  same  direction,  one  break-piece  only  is 
placed  on.  In  this  case,  the  circuit  is  interrupted  when  the  current 
changes,  that  is,  when  each  helix  quits  one  branch  of  the  magnet.  .  .  . 
On  placing  the  two  connecting  wires  R  S  between  M  N,  the  shock  is 
not  so  powerful.  TJ  and  V  (Fig.  102)  are  handles  connected  with  the 
conducting  wires,  and  furnished  with  pieces  of  sponge,  which  are  cm- 
ployed  in  the  application  of  electricity  for  medical  purposes.  These 
sponges  are  moistened  with  acid  or  saline  solutions.  By  means  of  them 
a  succession  of  the  most  powerful  shocks  may  be  applied  where  they  are 
needed.  ...  To  decompose  water,  Mr.  Clark  uses  the  apparatus 
(Fig.  104)  arranged  in  the  following  manner :  A  is  an  earthen  vessel 
with  a  brass  lid,  having  a  base  of  hard  wood,  through  which  pass  two 
copper  wires  soldered  to  platinum  wires,  and  which  are  connected  with 


ELECTRO-DYNAMICS. 


347 


M  N.  Two  tubes  A  are  filled  with  water,  and  then  placed  over  the 
platinum  wires,  where  they  are  supported  by  a  cork.  The  two  plates  of 
platinum  C  and  D,  which  are  connected  by  copper  wires  with  M  and  N, 


Fig.  104. 

are  for  showing  the  effects  of  electro-chemical  decompositions.  For  this 
purpose,  a  piece  of  litmus  or  turmeric  paper,  previously  moistened  with 
a  neutral  salt,  is  placed  between  the  disks.  In  the  place  of  the  two  pre- 


105. 


ceding  helices  and  their  accessories,  which  he  calls  the  intensity  arma- 
ture, because  the  current  obtained  is  from  electricity  of  high  tension,  Mr. 
Clark  employs  a  quantity  armature,  formed  of  less  powerful  cylinders, 
and  with  a  copper  wire,  covered  with  silk,  only  45  yards  long,  the  diam- 


348  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


eter  of  which  is  greater.     Fig.  105  represents  the  apparatus  furnished 
with  this  new  armature.     A  is  the  horseshoe  magnet,  D  the  armature, 
F  and  G  the  two  helices.     Attention  must  be  paid  that  the  spring  quits 
the  break-piece  at  the  moment  when  the  piece  is  vertical,  for  then  it  is 
that  it  is  in  a  neutral  position  relative  to  the  poles  of  the  magnet.     To 
fuse  iron  wire  with  bright  scintillations,  one  end  of  the  wire  is  connected 
with  the  end  P,  -and  the  other  end  is 
gently  pressed  on  the  rotating  armature        o 
D.     If  we  wish  to  obtain  sparks  of  dif- 
ferent colors  by  the  employment  of  dif- 
ferent metals,  the  break-piece  is  taken 
away,  and  the  piece  of  copper  B  (Fig. 
106)  is  substituted.     In  its  open  part  is 
introduced  a  piece  of  any  metallic  wire 
C,  gold,  for  example ;  the  extremity  of 
the  spring  G  is  also  of  gold.     On  mak- 
ing the  apparatus  rotate,  purple- colored 
sparks  are  obtained. 


Fig.  106. 


THERMO-ELECTRICITY. 

16.  The  electricity  which  is  developed  by  heat  is  called 
thermo-electricity.  When  two  different  metal  rods,  such  as 
copper  and  platinum,  or  antimony  and  bismuth,  are  soldered 
together,  and  heated  at  the  part  of  junction,  electricity  is  gen- 
erated. 

Exp.  1.  Twist  the  end  of  a  copper  wire  round  one  end  of  a  platinum 
wire ;  place  the  other  extremities  in  connection  with  the  binding  screws 
of  a  galvanometer;  heat  the  twisted  extremities  with  the  flame  of  a 
spirit  lamp ;  the  needle  of  the  galvanometer  will  be  instantly  deflected. 

Exp.  2.  Fix  two  copper  wires  into  the  binding  screws  of  a  galvanom- 
eter ;  heat  the  free  end  of  one  wire  with  the  flame  of  a  spirit  lamp ; 
bring  the  free  end  of  the  other  wire  into  contact  with  this  heated  wire ; 
the  needle  will  be  instantly  deflected,  thereby  showing  the  existence  of 
an  electric  current.  It  is  desirable  that  the  end  of  the  wire  which  is  to  be 
heated  should  terminate  with  a  small  plate. 

Exp.  3.  The  simple  apparatus  represented 
in  Fig.  107  exhibits  the  effects  of  thermo- 
electricity in  a  very  striking  manner,  a  b 
c  d  e  is  a  strip  of  copper,  bent  into  the  form 
shown  in  the  figure,  and  riveted  at  c.  A 


Fig.  107. 


ELECTRO-DYNAMICS.  349 

small  magnetic  needle  n  s  is  suspended  between  the  plates.  Heat  the 
free  end  a  of  the  copper  frame  with  the  flame  of  a  spirit  lamp,  and 
the  needle  will  be  instantly  deflected. 

Thermo-electric  Batteries. 

17.  These  batteries  are  formed  by  soldering  together  a  series  of  pairs 
of  metal  bars,  as  shown  in  Fig.  108,  where  the  dark  lines  represent  the 
bars  of  the  same  kind  of  metal,  and  the  faint  lines  those  of  the  other 
kind  of  metal.  Heat  is  applied  at  the  junctions  a  a  a,  while  the  junc- 
tions b  b  b  are  kept  cool.  The  extreme  ends  a  b  form  the  poles  of  the 

b    b    b    b    b    a  b      b        b        b      a 

'  VWW     UUTUTJ 

a    a     a    a     a  a        a        a         a 

Fig.  108. 

battery,  which  may  be  connected  with  binding  screws,  &c.  Eismuth 
and  antimony  are  the  two  metals  most  commonly  used  in  constructing 
these  batteries,  when  the  heat  employed  is  moderate ;  but  if  the  heat  to 
which  the  battery  is  to  be  exposed  is  great,  platinum  and  iron  should 
be  used. 

A  thermo-electric  battery  is  sometimes  used  as  a  thermometer.     Fig. 
109  represents  an  apparatus  of  this  kind,     a  the  tin  or  brass  box  which 
contains  the  thermo-battery  S,  com- 
posed of  bismuth  and  antimony  bars, 
arranged  according  to  the  principle  ex- 
plained in  connection  with  Fig.  108  ; 
m  and  p  the  binding  screws  connected 
with  the  poles  of  the  battery;  wires 
pass  from  these  binding  screws  to  the  Fig.  109. 

galvanometer  ;  b  and  c  are  the  two  lids 

of  the  box.  When  heat,  in  any  form,  is  applied  at  S,  the  deflection  of 
the  needle  indicates  the  degree  of  temperature  of  that  heat.  This  in- 
strument is  much  used  for  detecting  very  minute  differences  of  tempera- 
ture. A  good  instrument  will  readily  detect,  by  the  deflection  of  the 
needle,  a  difference  of  temperature  of  a  hundredth  part  of  a  degree. 


ACTION    OF    ELECTRO-MAGNETS     UPON    DIFFERENT     BODIES. 

18.   All  bodies  which  are  capable  of  being  magnetized  are 
called  magnetic  bodies  ;  but  Faraday  has  recently  shown  that 
30 


35Q          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

magnetism  everts  on  all  bodies,  more  or  less,  a  certain  pecu- 
liar influence,  very  different  from  the  magnetic  ;  those  bodies 
are  called  dia-magnetic.  Thus  the  flame  of  a  candle  under- 
goes a  peculiar  change  when  placed  between  the  poles  of  a 
powerful  magnet ;  and  light,  when  made  to  pass  over  the  poles 
of  a  magnet,  undergoes  a  change  of  polarity ;  and  so  on  to 
various  other  dia-magnetic  bodies. 


THE  ELECTRO-MAGNETIC  TELEGRAPH. 

19.  By  means  of  very  simple  expedients,  the  current  of  magnetism 
may  be  interrupted  hundreds  of  times  in  a  second,  being  fully  reestab- 
lished in  the  intervals.  These  effects  are  in  no  way  modified  by  the 
distance  of  the  place  of  interruption  of  the  current  from  the  magnet. 
Thus  pulsations  of  the  current  may  be  produced  by  an  operator  in  Eos- 
ton,  and  the  simultaneous  pulsations  of  the  magnetism  may  take  place 
at  New  Orleans,  provided  only  that  the  two  places  are  connected  by  a 
continuous  series  of  conducting  icircs. 

Now,  if  the  extremity  of  a  lever  which  is  attached  to  the  vibrating 
armature  carry  a  pencil  which  presses  upon  paper,  when  the  lever  is 
drawn  towards  the  electro-magnet,  and  if  at  the  same  time  the  paper  is 
moved  under  the  pencil  with  a  uniform  motion,  a  line  will  be  traced 
upon  the  paper  by  the  pencil,  the  length  of  which  will  be  proportionate 
to  that  of  the  interval  during  which  the  lever  is  held  in  contact  with 
the  stop.  As  the  operator  in  Boston  can  regulate  this  interval  at  will, 
by  controlling  the  flow  of  the  electric  current,  making  that  current  act 
for  a  short  interval  if  he  desire  to  make  a  short  line  upon  the  paper,  for 
a  long  interval  if  he  desire  to  make  a  long  line,  and  for  an  instant  if  he 
desire  to  make  merely  a  dot,  it  Mill  be  understood  how  he  can  at  will 
mark  a  sheet  of  paper  at  New  Orleans  with  any  desired  succession  of 
lines  of  various  lengths,  or  of  dots,  and  how  he  may  combine  these  in 
any  way  he  may  find  suitable  to  his  purpose. 


MORSE'S     TELEGRAPH. 

20.  This  apparatus,  which  is  applied  on  an  extensive  scale  in  Amer- 
ica, arid  with  some  slight  modifications  in  Germany,  is  constructed  upon 
the  principle  just  explained. 

A  general  view  of  the  instrument  in  its  most  usual  form  is  given  in 
the  following  figure. 


ELECTRO-DYNAMICS. 


351 


Fig.  110. 

M  is  the  electro-magnet ;  H  is  an  armature  working  on  the  centre  c ; 
i  is  an  adjusting  screw,  to  limit  the  play  of  the  armature  and  prevent  its 
contact  with  the  electro-magnet  at  p ;  d  is  another  adjusting  screw,  to  limit 
its  play  in  the  other  direction ;  t  a  metallic  style,  which  marks  by  pres- 
sure a  band  or  ribbon  of  paper  drawn  from  the  roll  R,  and  carried  between 
the  rollers  o  and  o' ;  P  the  ribbon  of  paper  discharged  from  the  rollers 
o  o1  after  being  impressed  by  t  with  the  telegraphic  characters  ;  I,  b,  &c., 
clockwork  from  which  the  rollers  o  o'  receive  their  motion,  by  which 
motion  the  ribbon  of  paper  is  drawn  from  the  roll  R ;  /  the  spring  which 
draws  the  arm  H  of  the  electro-magnet  from  the  armature ;  S  S  the 
upright  pieces  supporting  the  clockwork ;  B  B  the  base  supporting  the 
instrument ;  D  the  key  commutator,  by  which  the  current  transmitted 
along  the  line  wire  is  alternately  transmitted  and  suspended ;  mn,m'  »', 
wires  by  which  the  coil  of  the  electro-magnet  and  the  poles  of  the  sta- 
tion battery  are  put  in  connection  with  the  line  wires. 

The  following  are  the  telegraphic  characters  adopted  by  Professor 
Morse  for  the  English  language  :  — 


A-  — 

B 

C 

D 

E- 

F 

G 

H 

I  — 


J 

K 

L 

M  — 
N  — - 
O  -  - 

P 

Q 

R-  -- 


s... 

T  — 

IT 

•y 

w 

X 

Y 

Z---  - 


352 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Numerals. 


1 

2 

3 

4 

5 


BAIN'S    ELECTRO-CHEMICAL    TELEGRAPH. 

21.  The  chemical  properties  of  the  electric  current  can  be  made  to 
supply  the  means  of  transmitting  signals  between  two  distant  stations. 
When  a  current  of  adequate  intensity  is  made  to  pass  through  certain 
chemical  compounds,  it  is  found  that  these  are  decomposed,  one  of  their 
constituents  being  carried  away  in  the  direction  of  the  current,  and  the 
other  in  the  contrary  direction. 

Of  the  forms  of  telegraph  in  which  this  principle  is  brought  into  play, 
the  only  one  which  has  been  practically  applied  on  an  extensive  scale 
is  that  by  Mr.  Alexander  Bain. 


Fig.  111. 


To  understand  this  instrument,  let  us  suppose  a  sheet  of  writing  paper 
to  be  wet  with  a  solution  of  prussiate  of  potash,  to  which  a  little  nitric 
and  hydro-  chloric  acid  have  been  added.  Let  a  metallic  desk  be  pro- 


ELECTRO-DYNAMICS. 


353 


vided  corresponding  in  size  with  the  sheet  of  paper,  and  let  this  desk  be 
put  in  communication  with  a  galvanic  battery  so  as  to  form  its  negative 
pole.  Let  a  piece  of  steel  or  copper  wire,  forming  a  pen,  be  put  in  con- 
nection with  the  same  battery  so  as  to  form  its  positive  pole.  Let  the 
sheet  of  moistened  paper  be  now  laid  upon  the  metallic  desk,  and  let 
the  steel  or  copper  point  which  forms  the  positive  pole  of  the  battery  be 
brought  into  contact  with  it.  The  galvanic  circle  being  thus  completed, 
the  current  will  be  established,  the  solution  with  which  the  paper  is  wet 
will  be  decomposed  at  the  point  of  contact,  and  a  blue  or  brown  spot  will 
appear.  If  the  pen  be  now  moved  upon  the  paper,  the  continuous  suc- 
cession of  spots  will  form  a  blue  or  brown  line  ;  and  the  pen  being  moved 
in  any  manner  upon  the  paper,  characters  may  be  thus  written  upon  it, 
as  it  were,  in  blue  or  brown  ink. 

By  means  of  wheelwork,  the  metallic  desk  is  made  to  revolve  round 
its  centre  in  its  own  plane,  while  the  style  receives  a  slow  motion  directed 
from  the  centre  of  the  disk  towards  its  edge.  In  this  way  the  style 
traces  a  spiral  curve  upon  the  paper,  winding  round  it  continually,  and 
at  the  same  time  retiring  constantly  but  slowly  from  its  centre  towards 
its  edge.  It  will  be  evident,  without  further  explanation,  that  charac- 
ters may  thus  be  produced  on  the  prepared  paper  corresponding  to  those 
of  the  telegraphic  alphabet  already  described  in  Morse's  Telegraph. 


HOUSE'S   TELEGRAPH. 


30 


Fig.  112. 


354  NATURAL    AND    EXPEUniZlS'TAL    PHILOSOPHY. 

22.  This  apparatus,  which  is  in  extensive  use  in  the  United  States,  is 
a  printing  telegraph  —  that  is,  an  instrument  which  prints  in  the  ordi- 
nary letters  the  despatch  at  the  station  to  which  it  is  addressed,  by  means 
of  a  power  worked  at  the  station  from  which  it  is  transmitted.     It  con- 
sists of  two   distinct  parts  —  a  commutating  apparatus,  to  govern  the 
transmission  of  the  current,  and  a  printing  apparatus,  upon  which  the 
current  operates. 

The  transmission  of  the  current  is  controlled  by  the  keys  of  the  finger 
board.  The  wheel  that  produces  by  its  revolution  the  pulsations  of  the 
current  is  moved  by  the  foot  of  the  operator  acting  upon  a  treddle.  The 
rotation  of  this  wheel  is  arrested  at  the  point  corresponding  to  any  desired 
letter,  by  putting  down  with  the  finger  the  key  upon  which  that  letter 
is  engraved. 

23.  Mr.  Bernstein,  of  Berlin,  has  invented  a  modification  of  the  elec- 
tric telegraph,  which  promises  to  extend  the  advantages  of  that  machine 
in  a  remarkable  manner.     The  peculiarity  of  the  invention  is,  that  by 
one  wire  two  different  messages  can  be  sent  in  the  same  or  in  opposite 
directions  at  one  time.     Experiments  were  made  An  London  with  the 
new  machine,  and  they  are  said  to  have  fully  established  its  powers. 

TELEGRAPH  LINES  IN  THE  UNITED  STATES. 

24.  Owing  to  the  rapid  progress  and  unrestricted  freedom  of  enter- 
prise in  the  United  States,  a  great  number  of  independent  companies 
have  been  formed,  by  which  the  vast  territory  from  the  Atlantic  Ocean 
to  the  Mississippi,  and  from  the  Gulf  of  Mexico  to  the  frontiers  of  Can- 
ada, is  overspread  with  a  network  of  wires,  upon  which  intelligence  of 
every  description,  and  personal  and  commercial  correspondence,  are  flow- 
ing night  and  day,  incessantly,  from  year  to  year,  in  a  torrent  of  which 
the  old  continents  offer  no  similar  example. 

The  American  lines  are  generally  classified  according  to  the  telegraph 
instruments  with  which  they  work.  These  are  those  of  Morse,  House, 
and  Bain  ;  all  of  which  transmit  despatches  by  means  of  a  single  con- 
ducting wire,  and  all  of  which  write  or  print  the  despatches  they  trans- 
mit —  those  of  Morse  and  Bain  in  a  telegraphic  cipher,  and  that  of  House 
in  the  common  Roman  capitals. 

Of  these  three  systems,  that  of  Morse  is  in  the  most  general  use  —  a 
circumstance  which  is  partly  explained  by  the  fact  that  it  was  the  ear- 
liest adopted,  and  had  established  its  ground  long  before  either  of  the 
competing  systems.  It  must  be  admitted  that,  so  far  as  public  opinion 
and  favor  can  be  accepted  as  a  test  of  practical  excellence,  the  system  of 
Morse  has  received  not  only  a  large  majority  of  patronage  in  the  United 
States,  but  also  in  the  northern  and  eastern  states  of  Europe.  In  1854, 
the  total  extent  of  telegraphic  wire  then  in  operation  in  the  United  States 


ELECTRO-DYNAMICS.  355 

was  above  40,000  miles ;  of  which  Morse's  had  36,972  miles,  House's 
3850  miles,  and  Bain's  570  miles. 

The  most  distant  points  connected  by  electric  telegraph  in  North 
America  are  Quebec  and  New  Orleans,  which  are  3000  miles  apart ;  and 
two  separate  lines  connect  New  York  with  New  Orleans,  —  one  running 
along  the  seaboard,  the  other  by  way  of  the  Mississippi,  —  each  about 
2000  miles  long.  Messages  have  been  transmitted  from  New  York  to 
New  Orleans,  and  the  answers  received,  in  the  space  of  three  hours, 
though  they  had  necessarily  to  be  written  several  times  in  the  course  of 
transmission. 

The  electric  telegraph  is  used  by  all  classes  of  society  as  an  ordinary 
method  of  transmitting  intelligence. 

Government  despatches,  and  messages  involving  the  life  or  death  of 
any  persons,  are  entitled  to  precedence ;  next  come  important  press  com- 
munications ;  but  the  latter,  if  not  of  extraordinary  interest,  await  their 
regular  turn. 

Interruptions  occur  most  frequently  from  the  interference  of  atmos- 
pheric electricity ;  in  summer,  they  are  estimated  to  take  place,  on  an 
average,  twice  a  week.  Other  accidental  causes  of  interruption  occur 
irregularly,  from  the  falling  of  the  poles,  the  breaking  of  wires  by  fall- 
ing trees,  and,  particularly  in  winter,  from  the  accumulated  weight  of 
snow  or  ice. 

The  electric  current  is  made  to  act  through  long  distances  by  using 
local  and  branch  circuits  and  relay  magnets,  in  those  systems  where  it 
would  be  otherwise  too  weak  to  operate  effectually. 

No  adaptation  of  the  system  can  be  more  interesting  and  useful  than 
that  which  is  made  for  the  purpose  of  conveying  signals  of  alarm  and 
intelligence  in  the  case  of  fire.  This  has  been  completely  developed  in 
Boston.  The  city  is  divided  into  seven  districts,  each  provided  with  a 
powerful  alarm  bell.  Every  district  contains  several  stations  ;  there  are 
altogether  in  the  seven  districts  forty-two  stations,  all  of  which  are  con- 
nected with  a  chief  central  office,  to  which  intelligence  of  fire  is  con- 
veyed, and  from  which  the  alarm  is  given. 

At  each  of  the  stations  there  is  erected,  in  some  conspicuous  position, 
a  cast-iron  box  containing  the  apparatus  for  conveying  intelligence  to 
the  central  office  ;  and  by  striking  the  signal  bell  a  certain  number  of 
times,  the  district  and  station  from  which  the  signal  is  made  are  indi- 
cated. An  attendant  is  always  on  the  watch  at  the  central  office ;  and 
when  his  attention  is  called  to  the  signals  by  the  striking  of  a  large  call 
bell,  he  immediately  sets  in  motion  his  alarm  apparatus,  and,  by  depress- 
ing his  telegraph  key,  causes  all  the  alarm  bells  of  the  seven  districts  to 
toll  as  many  times  in  quick  succession  as  will  indicate  the  district  where 
the  fire  has  occurred  —  the  alarm  being  repeated,  at  short  intervals,  as 
long  as  may  be  necessary. 


ASTRONOMY. 


OBJECTS   OF  ASTRONOMY.— GENERAL  VIEW  OF   THE 
HEAVENS. 

1.  ASTRONOMY  is  that  science  which  treats  of  the  heavenly 
bodies  —  the  sun,  the  moon,  and  the  stars. 

THE    STARS. 

2.  When  we  look  at  the  heavens,  on  a  clear  night,  they 
appear  to  us  like  a  vast  dome,  or  concave  hemisphere,  in 
which  the  stars  shine  like  so  many  brilliant  gems  of  light. 


*• 


—• * 


JV^.  1.    The  Stars. 

The  point  directly  over  our  heads  is  called  the  zenith,  and 
the  line  where  the  sky  and  the  earth  appear  to  meet  is  called 

(356) 


ASTRONOMY.  357 

the  horizon.     The  nadir  is  that  point  in  the  heavens  which 
is  opposite  to  the  zenith ;  that  is,  it  lies  directly  below  our  feet. 

Thus  in  Fig.  1,  let  c  n  e  s  represent  the  earth,  .arid  c  the  place  of  a 
person  looking  at  the  stars ;  then  Z  is  his  zenith,  D  his  nadir,  and  H  R 
his  horizon ;  H  Z  R  is  the  hemisphere,  or  half  sphere,  of  stars  which 
are  visible  to  him,  and  H  D  E  the  opposite  hemisphere,  which  would  be 
visible  to  a  spectator  at  e  on  the  opposite  side  of  the  earth. 

In  the  daytime  we  do  not  see  the  stars  on  account  of  the 
superior  light  of  the  sun;  just  in  the  same  way  as  we  should 
not  see  the  flame  of  a  candle,  at  the  distance  of  a  few  hun- 
dred yards  from  us,  when  the  sun  is  shining ;  but  with  a  tel- 
escope the  stars  can  be  seen  at  any  time  of  the  day. 

CARDINAL    POINTS. 

3.  If  you  look  towards  the  sun  at  noon,  your  face  is  di- 
rected to  the  south  ;  your  back  is  towards  the  north ;  the  east 
is  on  your  left  hand ;  and  the  west  is  on  your  right.     These 
four  points  in  the  horizon  are  called   the    cardinal  points. 
Your  shadow  at  noon  is  shorter  than  it  is  at  any  other  time 
of  the  day,  because  the  sun  has  then  attained  his  greatest 
elevation  above  the  horizon.     The  sun  rises  towards  the  east, 
and  sets  towards  the  west.     At  noon  the  sun  is  said  to  be  on 
the  meridian  ;  and  the  time  which  elapses  between  his  leav- 
ing the  meridian  and  returning  to  it  again  is  called  a  solar  day. 

DIURNAL    MOTION    OF    THE   HEAVENS. 

4.  If  we  look  attentively  at  the  stars,  on  a  cloudless  night, 
we  shall  see  one  star  after  another  rising  above  our  horizon  in 
the  east,  and  star  after  star  setting,  or  sinking  beneath  our 
horizon  in  the  west.     A  little  farther  observation  will  show 
us  that  the  whole  visible  heavens  appear  to  turn  from  east  to 
west  about  a  certain  little  star,  considerably  elevated,  called 
the  polar  star  ;  and  that  a  complete  revolution  is  made  in  the 
course  of  every   day.     Now,   this   apparent   motion   of  the 
heavens,  as  we  shall  afterwards  see,  is  really  produced  by  the 


358          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

revolution  or  turning  of  the  earth  from  west  to  east,  round  a 
line  or  axis,  which  we  may  conceive  to  be  drawn  through  the 
centre  of  the  earth  and  the  polar  star.  This  is  called  the 
diurnal  motion  of  the  heavens,  because  it  is  performed  in  the 
course  of  a  day. 

•  In  Fig.  1,  N  represents  the  north  polar  star,  N  S  the  Hue  round  which 
the  heavens  appear  to  turn,  or  the  line  round  which  the  earth  really 
turns. 

MAGNITUDE    OP   THE    STARS. 

5.  In  a  clear  night  about  two  thousand  stars  may  be  seen 
with  the  naked  eye,  but  with  a  small  telescope  many  millions 
may  be  observed.     The  stars  appear  to  us  of  different  sizes 
and  degrees  of  brightness ;  the  largest  and  brightest  are  said 
to  be  of  the  first  magnitude  ;  the  next  in  order  of  the  second 
magnitude ;  and  so  on  to  the  sixth  magnitude,  which  com- 
prises those  very  small  stars  which  are  just  visible  to  the 
naked  eye.     There  are  only  eleven  stars  of  the  first  magni- 
tude in  our  hemisphere,  and  six  in  the  southern,  or  opposite 
hemisphere.     There  are  about  fifty  of  the  second  magnitude, 
visible  to  us,  and  not  less  than  one  hundred  and  twenty  of  the 
third  magnitude. 

FIXED    STARS    AND    PLANETS. 

6.  Nearly  all  the  stars  which  we  see  are  fixed ;  that  is  to 
say,  they  do  not  change  their  distances  from  one  another,  but 
always  present  the  same  outline  of  form.     Some  of  the  stars, 
however,  do  not  always  remain  in  the  same  place,  but  move 
among  the  fixed  stars:  these  stars  are  called  planets. 

The  fixed  stars  are  also  distinguished  from  the  planets  by 
having  a  more  twinkling  sort  of  light ;  and  viewed  through  a 
telescope,  the  planets  look  like  little  luminous  globes,  while 
the  stars  simply  appear  like  brilliant  points  of  light  without 
any  appreciable  size. 


CONSTELLATIONS. 

7.  The  ancient  astronomers,  for  the  convenience  of  refer- 
ence, formed  the  fixed  stars  into  constellations,  or  groups  of 
stars,  and  represented  them  by  animals,  and  other  things,  to 
which  they  imagined  the  outline  of  the  stars,  in  each  group, 
had  some  resemblance.  The  most  striking  of  the  constella- 
tions is  that  of  the  Great  Bear,  which  is  commonly  known  by 
the  name  of  Charles's  Wain,  or  wagon.  Sailors  call  it  the 
dipper. 

The  form  of  this  constellation  is  shown  in  Fig.  2,  where  the  four  stars 
abed  are  supposed  to  represent  the  body  of  the  dipper,  and  the  remain- 
ing three  the  handle.  The  two  stars  b  a  are  called  the  pointers  ;  for  if  a 
line  be  drawn  through  them  it  will  very  nearly  point  to  the  polar  star  N. 


Fig.  2.     Constellation  of  the  Great  Bear. 


If  a  line  be  drawn  from  the  star  e,  leaving  v  a  little  to  the  left,  it  will 
pass  through  a  very  brilliant  star  A,  called  Arcturus,  which  is  the  prin- 
cipal star  in  the  constellation  of  Boo'tes. 

The  names  of  the  different  constellations  may  be  readily 
acquired  by  looking  at  a  celestial  globe,  which  is  constructed 
to  represent  the  aspect  of  the  heavens.  These  constellations 
always  present  the  same  appearance ;  the  hoary-headed  man, 
tottering  on  his  grave,  as  he  takes,  it  may  be,  a  last  look  at 
Charles's  Wain,  well  remembers  that  it  presented  the  same 
aspect  when  he  first  gazed  upon  it  in  the  joyous  days  of  his 
childhood. 


360          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 
SIGNS    OF   THE    ZODIAC. THE    ECLIPTIC. 

8.  There  is  a  remarkable  class  of  constellations,  extending 
round  the  heavens,  like  a  band  or  belt,  in  which  the  planets 
always  appear  to  move ;  this  belt  of  stars  contains  twelve 
constellations,  which  are  called  the  signs  of  the  zodiac.  The 
sun  also  appears  to  us  to  make  a  complete  revolution  in  the 
heavens,  in  the  course  of  a  year,  through  the  different  con- 
stellations of  the  zodiac.  This  apparent  path  of  the  sun  in 
the  heavens  is  called  the  ecliptic ;  the  constellations  of  the 
zodiac,  therefore,  mark  out  the  ecliptic  in  the  heavens.  The 
term  zodiac  means  animal,  and  this  apparent  path  of  the  sun 
was,  no  doubt,  so  called  on  account  of  the  names  given  to  the 
various  constellations  composing  it.  The  zodiac  is  divided 
into  twelve  signs,  to  correspond  to  the  twelve  months  of  the 
year.  The  following  table  gives  the  names  of  the  signs  of 
the  zodiac,  with  the  marks  or  symbols  which  are  put  for  them. 

Names  of  the  Signs  of  the  Zodiac. 


Aries    .  .  the  Earn 

Taurus  .  the  Bull . 

Gemini  .  the  Twins 

Cancer .  .  the  Crab 

Leo .     .  .  the  Lion . 

Virgo    .  .  the  Virgin 


Libra    .     .     the  Balance .     .  -^ 

Scorpio      .     the  Scorpion     .  [fl. 

Sagittarius     the  Archer   .     .  / 

Capricornus   the  Goat      .     .  ][f 

Aquarius  .     the  Waterman .  x& 

Pisces   .     .     the  Fishes   .     .  H 


GENERAL    PRINCIPLES    OF   ASTRONOMY. 

9.  In  the  study  of  astronomy,  it  is  above  all  things  neces- 
sary that  we  should  reason  upon  appearances,  and  that  we 
should  allow  the  first  rude  notions,  derived  from  the  senses,  to 
be  corrected  by  the  judgment.  As  these  remarks  are  essen- 
tial to  a  right  appreciation  of  our  methods  of  exposition,  it 
will  be  instructive  to  elucidate  them  by  taking  one  or  two 
familiar  cases. 

The  cross  at  the  top  of  St.  Paul's  cathedral  appears  to  us  not  longer 
than  a  walking  stick,  whereas  its  length  is  really  greater  than  the  clcva- 


ASTRONOMY.  361 

tion  of  an  ordinary  house.  Here,  by  taking  into  account  the  distance 
of  the  cross  from  us,  we  are  able  to  assign  a  reason  for  its  apparent 
smallness.  In  precisely  the  same  way,  the  moon  appears  to  us  scarcely 
larger  than  a  man's  face ;  but  when  we  consider  that  it  is  many 
thousands  of  miles  from  us,  we  should  be  prepared  for  adopting  the  fact 
that  it  is  a  world  not  much  smaller  than  the  earth  on  which  we  live. 
In  like  manner,  we  should  be  led  to  expect  that  the  planets,  which, 
owing  to  their  still  greater  distances  from  us,  appear  like  little  balls  of 
light,  are  in  reality  vast  globes,  many  of  which  are  considerably  larger 
than  our  earth. 

When  we  are  moving  in  a  railway  carriage,  we  should  from  appear- 
ances believe,  if  our  reason  did  not  correct  this  belief,  that  the  houses 
and  trees  were  moving,  and  that  we  were  sitting  still.  In  like  manner, 
we  should  be  prepared  to  question  the  truth  of  the  first  impression  of 
our  senses,  when  we  are  led  to  imagine  that  the  whole  of  the  heavens 
turn  round  us  in  every  twenty-four  hours,  and  to  ask  ourselves,  "  Is  it 
not  more  rational  to  suppose  that  this  apparent  motion  is  produced  by 
the  actual  rotation  of  our  earth  itself? "  The  science  of  astronomy  has 
established  many  principles  which  are  at  variance  with  the  first  rude 
notions  derived  from  mere  appearances  :  the  following  general  principles 
deserve  especial  attention  :  — 

Our  earth  has  the  form  of  a  globe ;  it  turns  or  spins  round 
upon  its  axis  every  twenty-four  hours,  and  thus  gives  rise  to 
the  apparent  diurnal  or  daily  motion  of  the  heavens  ;  it  also 
moves  round  the  sun,  in  the  course  of  a  year,  which  occa- 
sions the  apparent  motion  of  the  sun  in  the  ecliptic.  The 
planets  are  worlds  like  our  own ;  and  they,  together  with  our 
earth,  revolve  round  the  sun  as  the  common  centre  of  attrac- 
tion, in  different  paths  or  orbits  ;  they  also  derive  their  light 
and  heat  from  it.  The  sun,  with  all  the  planetary  bodies 
which  move  round  it  as  a  centre,  is  called  the  solar  system. 
The  fixed  stars,  which  are  at  immense  distances  from  us,  are 
suns,  with  their  respective  systems  of  unseen  worlds  revolving 
round  them,  probably  similar  to  the  solar  system. 
31 


362         NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


SOLAR   SYSTEM. 

10.  The  solar  system  consists  of  the  sun,  in  the  centre, 
round  which  all  the  planetary  bodies  revolve. 

The  leading  planets  move  round  the  sun  from  west  to  east, 
in  nearly  circular  orbits  or  paths,  lying  nearly  in  the  same 
plane,  or  flat  surface,  that  is,  in  the  plane  of  the  ecliptic,  and 


Fig.  3.    Solar  System. 

rotate,  or  spin  round  on  their  axes,  in  the  same  direction. 
Some  of  the  planets  have  moons  or  satellites  revolving  round 
them.  The  names  of  the  planets,  in  the  order  of  their  dis- 
tances from  the  sun,  are  MERCURY,  VENUS,  the  EARTH, 
MARS,  JUPITER,  SATURN,  URANUS,  and  NEPTUNE  ;  together 
with  ten  small  planets  called  ASTEROIDS,  or  little  stars,  which 
move  in  orbits  lying  between  Mars  and  Jupiter.  These  are 


ASTRONOMY.  363 

called  primary  planets  ;  there  are  also  20  moons  or  satellites, 
which  are  called  secondary  planets,  because  they  revolve  round 
their  respective  primaries  in  the  same  manner  as  the  latter 
revolve  round  the  sun.  The  moon  is  the  satellite  to  the 
earth:  it  completes  a  revolution  round  the  earth  in  the 
course  of  every  lunar  month.  Jupiter  has  four  satellites, 
Saturn  eight,  Uranus  six,  and  Neptune  one.  Besides  these, 
there  is  another  order  of  bodies,  which  revolve  round  the  sun 
called  comets ;  they  have  blazing  trails,  and  move  in  very 
eccentric  orbits. 

The  solar  system,  as  just  described,  was  first  taught  by  Pythagoras, 
an  eminent  Greek  philosopher,  who  lived  about  §00  years  before  the 
time  of  Christ.  But  it  was  soon  after  disregarded,  and  various  false  sys- 
tems were  taught  in  its  place,  until  about  300  years  ago,  when  Coper- 
nicus revived  the  true  system  which  had  been  discovered  by  the  great 
Pythagoras. 

The  planets,  with  the  other  bodies  composing  the  solar  system,  will  be 
more  fully  described  after  we  have  considered  the  different  motions,  &c., 
of  the  earth  and  the  moon. 


THE  EARTH  AND  ITS  MOTION. 

FORM   AND    SIZE    OF   THE    EARTH. 

11.   The  earth  has  the  form  of  a  globe  ;  that  is,  it  is  like 


%; 

^--^Sr 

'"=Mrp^^-Z 

Fig.  4.    The  Earth  in  Space. 


364          NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

a  ball  or  orange. '    This  is  proved  by  various  facts ;  the  fol- 
lowing are  given  as  being  the  most  simple. 
Navigators  have  sailed  round  the  earth. 

If  a  ship  sail  constantly  in  the  same  general  direction,  either  eastward 
or  westward,  she  will  arrive  at  the  same  place  from  which  she  set  out. 
Now,  if  the  earth  were  an  unbounded  plain,  or  flat  surface,  the  farther 
the  ship  sailed,  the  farther  she  would  get  from  the  point  of  departure. 
Magellan  was  the  first  mariner  that  sailed  round  the  world,  but  Colum- 
bus was  the  first  that  made  the  attempt. 

The  earth,  then,  is  a  great  globular  mass  of  matter,  without  any  fixed 
point  of  support ;  for  navigators  and  travellers  have  crossed  it  in  all 
directions,  and  no  such  point  has  ever  been  seen. 

The  hull  of  a  vessel  disappears  as  she  leaves  the  shore. 

When  a  vessel  leaves  the  shore,  at  a  little  distance,  a  portion  of  the  hull 
is  observed  to  disappear ;  a  little  farther,  the  hull  is  lost  to  the  sight ; 
at  a  greater  distance,  the  lower  sails  disappear ;  until  at  length,  only  the 


Fig.  5.     Rotundity  or  Roundness  of  the  Earth. 

upper  sails  are  seen  in  the  horizon,  or  the  line  where  the  earth  and  sky 
appear  to  meet.  But  if  we  now  ascend  a  high  tower,  we  should  get 
sight  of  the  hid§ngain.  Now,  if  the  earth  were  a  flat  surface,  we  should 
always  see  th,e  hull  at  the  same  time  that  we  see  the  topsails. 

The  earth  always  appears  of  a  circular  shape. 

The  rotundity  or  roundness  is  such,  that  a  man  six  feet  high,  standing 
upon  the  sea  shore,  would  see  a  little  boat  when  its  distance  from  him 
does  not  exceed  three  miles ;  but  if  he  were  elevated  twenty-four  feet, 
the  boat  wrould  be  seen  at  the  distance  of  six  miles ;  and  if  he  were 
elevated  fifty-four  feet,  the  boat  would  be  seen  at  the  distance  of  twenty- 
seven  miles  ;  and  so  on :  the  distance  at  which  the  boat  would  be  seen 
increasing  with  the  elevation  of  the  observer.  In  all  these  cases,  the 
man's  view  is  bounded  by  a  circular  horizon.  Now,  there  is  no  body  but 
a  globe  that  will  always  appear  of  a  circular  shape  when  viewed  at  dif- 
ferent distances. 


ASTRONOMY. 


365 


*****  


Fig.  6.    A  Globe  always  appears  round. 

At  whatever  distance  I  look  at  this  little  globe,  it  always  appears  to 
have  a  circular  shape  ;  and  moreover,  the  farther  it  is  removed  from  my 
eye,  the  greater  is  the  extent  of  surface  that  I  see.  Mr.  Green,  when  he 
goes  up  with  his  balloon,  will  see  more  of  the  earth's  surface  than  we 
can,  even  though  we  should  be  on  the  top  of  Richmond  Hill ;  and  what- 
ever may  be  his  height  above  the  earth's  surface,  he  will  always  find  that 
it  presents  a  circular  shape.  When  he  has  attained  his  greatest  eleva- 
tion, the  largest  hills  and  trees  will  appear  to  him  just  like  the  little 
irregularities  which  we  see  upon  the  surface  of  an  orange. 


Fig.  7.     The  Earth  always  appears  round. 


THE  DIAMETER  OF  THE  EARTH. 

12.  The  diameter,  or  line  passing  through  the  centre  ol  the 
earth,  is  about  8000  miles ;  and  as  the  length  of  a  line  going 
round  a  circle  is  a  little  more  than  three  times  the  diameter, 
it  follows  that  the  length  of  a  line  going  round  the  earth,  or 
the  circumference,  is  about  25,000  miles.  A  railway  train, 
moving  with  the  speed  of  50  miles  per  hour,  would  go  round 
the  earth  in  about  500  hours,  or  about  three  weeks,  supposing 
there  were  no  obstructions  to  the  motion.  This  will  give  us 
some  idea  of  the  great  size  of  the  earth. 
31* 


366          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


DIURNAL    MOTION    OF   THE    EARTH. 

13.  The  earth  has  two  motions  —  &  diurnal  or  daily  mo- 
tion upon  its  axis,  and  an  annual  or  yearly  motion  round  the 
sun ;  that  is,  it  turns  round  like  a  spinning  top,  and  at  the 
same  time  moves  round  the  sun. 

14.  Cause  of  Day  and  Night.  —  The  spinning  motion,  or 
revolution  of  the  earth  upon  its  axis,  is  the  cause  of  day  and 
night.     When  the  sun  shines  upon  our  side  of  the  earth,  it  is 
day  with  us,  and  when  he  shines  on  the  opposite  side,  it  is 
night  with  us. 

If  you  hold  a  globe  or  orange  before  a  candle,  one  half  of  the  globe 
will  be  enlightened,  and  the  other  half  will  be  in  the  shade ;  and  if  the 
globe  be  turned  round,  every  portion  will  be  successively  brought  within 
the  light  of  the  candle.  The  line  ef,  separating  the  light  and  shade,  is 
called  the  circle  of  illumination.  Let  us  suppose  a  little  fly  to  be  fixed 
on  this  globe ;  then,  throughout  one  half  of  the  revolution,  the  creature 
will  be  in  the  shade,  and  throughout  the  other  half,  the  creature  will  be 
in  the  light.  When  the  fly  comes  on  the  circle  of  illumination,  it  will 
then  just  begin  tcwsee  the  candle ;  and  when  it  is  passing  out  of  the 
circle  of  illumination,  on  the  other  side,  the  candle  will  just  be  disap- 


Fig.  8.    Light  and  Shade. 

pearing  to  it ;  but  when  it  is  in  the  middle  of  these  two  points,  the  can- 
dle will  shine  directly  or  perpendicularly  over  it,  and  here  it  will  enjoy 
the  greatest  amount  of  light  and  heat  from  the  candle.  So  it  is  with 
our  earth ;  the  sun  enlightens  one  half  of  the  earth  at  one  time,  the  other 
half  being  in  darkness.  When  a  place  just  comes  within  the  circle  of 
illumination,  the  sun  then  begins  to  shine  or  rise  to  that  place  ;  and  on 
the  contrary,  when  the  place  is  just  going  out  of  the  circle  of  illumina- 
tion, the  sun  will  be  disappearing  or  setting  to  that  place  ;  and  midway 
between  these  two  lines  of  illumination,  the  sun  will  shine  directly  over, 
or  perpendicularly  over  the  place,  and  then  it  will  be  noon  to  that 
place. 


ASTRONOMY. 


3G7 


Now,  if  the  earth  were  standing  still,  one  half  of  it  would  have  per- 
petual day,  and  the  other  half  would  have  perpetual  night.  But  in 
order  that  the  whole,  or  nearly  the  whole,  of  the  earth  may  be  habitable, 
it  is  ordained  by  our  good  and  all- wise  Creator,  that  the  earth  should 
turn  round  once  every  natural  day,  so  that  every  portion  might  enjoy,  in 
succession,  the  light  and  heat  of  the  sun.  This  motion  of  the  earth  is 
called  the  diurnal  or  daily  motion. 

15.  But  how,  it  may  be  asked,  do  we  think  that  the  sun  and  stars 
move  from  east  to  west  ?  Just  in  the  same  way  as  when  we  are  in  a 
railway  carriage  we  believe,  if  our  reason  were  not  to  correct  the  belief, 
that  the  nearest  trees  and  houses  have  a  motion  contrary  to  that  which 
we  really  have. 

In  order  to  illustrate  this  still  further :  let 
A  represent  an  object  capable  of  moving 
round  the  globe  E  F,  which  admits  of  turn- 
ing on  its  axis  o.  First  let  the  object  A 
move  round  the  globe  in  the  direction  of  the 
arrow  shown  in  the  figure,  while  the  globe 
itself  remains  at  rest ;  the  object  at  A  will 
appear  in  the  horizon  to  a  spectator  at  E  ; 
but  as  the  object  moves,  it  will  appear  to  the 
spectator  to  rise  higher  and  higher  above  the 
horizon,  until  it  arrives  at  B,  when  it  will 
appear  in  the  zenith,  or  directly  over  the 

head  of  the  person.  Next  let  the  globe  turn  round  on  its  axis  o,  in  a 
direction  contrary  to  that  in  which  the  object  moved,  as  shown  by  the 
arrow  in  the  figure,  while  the  object  A  stands  still ;  the  apparent  motion 
of  the  object,  as  seen  by  the  spectator  at  E,  will  be  exactly  the  same  as 
before ;  thus,  when  the  globe  begins  to  revolve,  the  object  A  will  appear 
to  the  spectator  E  to  be  in  the  horizon  ;  but  as  the  globe  turns  round, 
the  object  A  will  appear  to  rise  higher  and  higher  above  the  horizon, 
until  the  spectator  has  turned  round  to  F,  when  the  object  will  appear 
in  his  zenith,  or  directly  over  his  head.  Now,  if  the  globe  tunied  round 
on  its  axis  without  any  jarring  motion,  or  without  any  jolting  or  shak- 
ing, as  the  earth  really  does,  so  that  our  spectator  might  be  altogether 
insensible  of  his  own  motion,  then  it  is  plain  that  he  would  at  first  be- 
lieve that  the  object  had  moved  from  A  to  B,  that  is,  from  his  horizon  to 
his  zenith,  in  the  place  of  having  himself  moved  round  with  the  globe 
from  E  to  F.  Thus  the  apparent  motion  of  the  heavens,  from  east  to 
west,  would  be  produced  by  the  actual  rotation  of  the  earth  on  its  axis 
from  west  to  east. 

It  would  be  opposed  to  the  simplicity  which  we  every  where 
observe  in  the  works  of  God,  as  well  as  at  variance  with  the 


368 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


known  laws  of  mechanics,  to  suppose  that  the  sun,  with  many 
thousands  of  worlds,  most  of  which  are  vastly  larger  than  our 
own,  should  move  round  our  globe  once  in  every  day,  when 
the  same  end  could  be  served  by  our  earth  simply  turning  on 
its  axis.* 

LINES    UPON    THE    GLOBE. 

16.  The  earth,  then,  makes  a  complete  revolution  every 
day ;  the  line  about  which  it  turns  is  called  the  axis  of  the 
earth  ;  and  the  points  where  this  imaginary  axis  pierces  the 
earth's  surface  are  called  the  poles  ;  there  are,  therefore,  two 
poles,  the  one  being  called  the  north  pole,  the  other  the 
south  pole.  If  a  line  be  drawn  round  the  earth,  every  where 
at  the  same  distance  from  the  two  poles,  it  will  form  the 
equator. 

If  you  spin  a  globe  upon  its  axis,  you  will  find  that  the  line  which  we 
call  the  equator  has  the  quickest  motion,  and  that  the  poles  are  the  only 

North  Pole. 

H 


South  Pole. 
Fig.  10.    The  Globe. 

points  on  the  surface  of  the  globe  which  have  no  motion.  In  this  figure, 
N  S  represents  the  axis  of  the  earth,  N  the  north  pole,  S  the  south  pole, 
E  Q  the  equator. 

*  The  rotation  of  the  earth  has  recently  been  proved  by  an  experiment 
with  a  long  pendulum. 


ASTRONOMY.  369 

The  equator,  therefore,  divides  the  earth  into  two  equal 
parts ;  the  portion  E  N  Q  is  called  the  northern  hemisphere, 
or  half  sphere;  and  the  portion  E  Q  S  is  called  the  southern 
hemisphere* 

Circles  upon  the  globe  are  divided  into  360  equal  parts, 
and  each  part  is  called  a  degree.  A  semicircle,  or  half  circle, 
will  contain  180  degrees,  (180°.)  A  quadrant,  or  quarter 
circle,  will  contain  90  degrees,  (90°.)  The  distance  of  the 
equator  from  either  of  the  poles  will,  therefore,  contain  90°. 


LATITUDE    AND    LONGITUDE. 

17.  A  line  drawn  between  the  north  and  south  poles  is 
called  a  meridian,  because,  when  any  meridian  is  opposite  to 
the  sun,  it  is   midday,  or  noon,  to  all   places  on  that  line. 
These  circles  will  all,  evidently,  lie   due  north  and  south. 
Meridian  lines  are  also  called  lines  of  longitude.     The  merid- 
ian passing  through  Greenwich  is  called  the  first  meridian,  or 
the  one  to  which  the  position  of  all  the  others  is  referred. 

18.  The  longitude  of  a  place  is  its  distance,  in  degrees, 
east   or  west,  from   the   first  meridian.     Thus  America  has 
west  longitude,  whereas  Asia  and  Africa  have  east  longitude. 

19.  The  latitude  of  a  place  is  its  distance  from  the  equa- 
tor.    All  places  in  the  northern  hemisphere  have  north  lati- 
tude ;  and  on  the  contrary,  all  places  in  the  southern  hemi- 
sphere have  south  latitude.     Thus  a  place  midway  between 
the  equator  and  north  pole  will   have   45°  north  latitude ; 
whereas  a  place  midway  between  the  equator  and  the  south 
pole  will  have  45°  south  latitude.     London,  being  51£°  from 
the  equator,  has  5l£°  north  latitude. 

20.  Lines  drawn  round  the  earth  parallel  to,  or  even  with, 
the  equator,  are  called  parallels  of  latitude.^    These  lines  are 


*  In  giving  these  lessons,  it  is  desirable  that  the  teacher  should  be  pro- 
vided with  a  small  white  globe,  having  a  rod  passing  through  it  to  represent 
the  axis  of  the  earth,  and  having  also  all  the  essential  lines  upon  the  terres- 
trial globe,  painted  in  strong  black  lines. 


370  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

called  small  circles,  because  they  are  less  than  the  great  circles, 
or  circles  which  divide  the  globe  into  two  equal  parts,  like  the 
equator.  The  use  of  parallels  of  latitude  is  to  point  out 
places  that  have  the  same  latitude  or  distance  from  the  equa- 
tor. The  latitude  of  a  place  will  obviously  be  measured  upon 
the  meridian  passing  through  the  place. 

21.  In  order  to  fix  the  exact  position  of  a  place,  we  must  have  its 
distances  from  two  known  lines.     Thus  my  position  in  this  room  will  be 
known  when  I  tell  you  that  I  am  twelve  feet  from  the  wall  in  front  of 
me,  and  ten  feet  from  the  wall  to  the  right  of  me.     So  it  is  with  respect 
to  the  earth ;  when  we  know  the  distance  of  a  place,  north  or  south, 
from  the  equator,  and  at  the  same  time  its  distance,  east  or  west,  from 
the  first  meridian,  the  position  of  that  place  becomes  known.     A  merid- 
ian, drawn  through  a  place,  will  cut  the  equator  in  a  certain  point,  the 
distance  of  which  from  the  first  meridian,  measured  in  degrees  on  the 
equator,  will  give  the  longitude  of  the  place ;  and  the  distance  of  the 
place  from  the  equator,  measured  in  degrees  upon  the  meridian,  will  give 
the  latitude.     Thus,  if  the  meridian  passing  through  the  place  lies  23° 
to  the  east  of  the  first  meridian,  then  the  place  will  have  23°  east  longi- 
tude ;  and  if  the  place  be  40°  north  from  the  equator,  the  latitude  will 
be  40°  north. 

22.  Because  the  earth  turns  once  on  its  axis  from  west  to  east,  or  de- 
scribes 360  degrees  in  the  course  of  twenty-four  hours,  it  follows  that 
the  twenty-fourth  part  of  360°,  or  15°,  will  be  turned  round  every  hour. 
A  place,  therefore,  having  15°  east  longitude,  will  have  noon  one  hour 
before  us ;  and  a  place  having  15°  west  longitude  will  have  noon  one 
hour  after  us.     In  general,  we  must  allow  an  hour  as  the  difference  of 
time  between  any  two  places  for  every  15°  difference  of   longitude. 
Thus  Alexandria  has  30°  east  longitude ;  consequently  as  many  times 
as  15°  can  be  taken  out  of  30°,  so  many  hours  will  the  people  of  this 
place  have  noon  before  us  in  London ;  that  is,  their  noon  will  take  place 
two  hours  before  our  noon. 

23.  By  this  means,  seamen  are  enabled  to  find  their  longitude  :  sup- 
pose, for  example,  that  the  pointer  of  the  clock  which  they  take  with 
them,  keeping  Greenwich  time,  should  be  at  nine  o'clock  in  the  morn- 
ing, when"  it  is  noon  to  the  place  of  observation ;  then  the  difference  of 
time  being  three  hours,  the  difference  of  longitude  will  be  three  times 
15°,  or  45°  ;  but  as  the  place  of  observation  has  noon  before  us,  it  will 
consequently  have  45°  east  longitude. 

24.  As  the  length  of  the   parallels   of  latitude   become 
shorter  and  shorter  as  they  approach  the  pole,  it  follows  that 


ASTRONOMY.  371 

a  degree  of  longitude,  estimated  on  any  parallel  of  latitude,  is 
shorter  than  a  degree  on  the  equator.  This  principle  is  ob- 
served in  the  construction  of  maps. 

THE    TROPICS    AND    ECLIPTIC. 

25.  If  the  sun  were  always  shining  perpendicularly  over 
the  equator,  as  in  Fig.  11,  the  length  of  the  day  and  night 
would  always  be  equal  all  over  the  globe.  The  sun  has  this 
position  at  the  commencement  of  our  spring  and  autumn,  that 
is,  on  the  21st  of  March  and  on  the  22d  of  September. 


Fig.  11.     The  Sun  in  Spring  and  Autumn. 

Owing  to  causes  which  will  afterwards  be  explained,  we  find  that 
during  our  midsummer  day,  the  sun  shines  perpendicularly  over  a  line 
c  v,  going  round  the  earth  23^°  on  the  northern  side  of  the  equator. 
(See  Figs.  11,  12.)  This  line  is  called  the  tropic  of  Cancer,  because 
the  sun  appears  to  us,  at  this  time,  amongst  a  certain  group  of  stars 


/  8 

Fig.  12.    The  Sun  in  Summer. 

called  the  constellation  of  Cancer,  or  the  Crab.  Now,  as  the  sun  en- 
lightens one  half  of  the  globe  at  one  time,  it  follows  that  his  light  must 
extend  23£°  over  the  north  pole,  that  is,  to  the  point  e  in  the  figure,  and 
a  line  e  d,  drawn  round  the  earth  parallel  to,  or  even  with,  the  equator, 
is  called  the  arctic  circle. 


372          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

In  like  manner,  during  our  midwinter  day,  (see  Fig.  13,)  the  sun 
shines  perpendicularly  over  a  line  g  p>  23£°  on  the  south  side  of  the 


Fig.  13.     The  Sun  in  Winter. 


equator ;  this  line  is  called  the  tropic  of  Capricorn,  because  the  sun 
appears  to  us,  at  this  time,  amongst  a  group  of  stars  called  the  constella- 
tion of  Capricorn,  or  the  Goat ;  and  the  line  f  q  in.  the  figure,  drawn 
round  the  earth  at  the  distance  of  23£°  from  the  south  pole,  is  called  the 
antarctic  circle. 

If  a  line  c  p  be  now  drawn  round  the  earth  between  the  tropics  of 
Cancer  and  Capricorn,  it  will  form  the  ecliptic  or  apparent  path  of  the 
sun  throughout  our  year.  The  ecliptic  is,  therefore,  inclined  to  the 
equator  at  an  angle  of  23£°. 

THE    ZONES    ON   THE    EARTH. 

26.  To  mark  out  the  climates  upon  the  earth,  its  surface  is 
divided  into  five  zones  or  belts.  The  portion  lying  between 
the  tropics  of  Cancer  and  Capricorn  is  called  the  torrid 
zone,  or  hot  zone ;  for  here  the  sun,  shining  almost  perpen- 
dicularly upon  the  earth,  will,  in  general,  cause  this  portion 
to  be  very  warm.  The  portion  in  the  northern  hemi- 
sphere lying  between  the  tropic  of  Cancer  and  the  arctic 
circle  is  called  the  north  temperate  zone ;  and  the  corre- 
sponding portion  in  the  southern  hemisphere,  lying  between 
the  tropic  of  Capricorn  and  the  antarctic  circle,  the  south 
temperate  zone.  The  surface  within  the  arctic  circle  is 
callecl  the  north  frigid  zone,  and  that  within  the  antarctic 
circle,  the  south  frigid  zone  ;  because,  from  the  slanting 
direction  with  which  the  sun's  rays  meet  the  surface  of  the 
earth  at  these  regions,  they  are  found  to  be,  in  general,  very 
cold. 


ASTRONOMY.  373 

27.  It  is  obvious  that  the  only  places  on  the  earth  to  which  the  sun 
can  be  vertical  are  those  lying  within  the  torrid  zone  ;  and  that  to  all 
such  places  there  can  be  but  little  variation  in  the  length  of  the  days. 
Whereas  within  the  frigid  zones  the  sun  will,  shine  for  a  certain  series 
of  days  without  setting,  and  for  a  corresponding  number  of  days  he  will 
not  appear  above  the  horizon. 

28.  The  elevation  of  the  polar  star  is  equal  to  the  latitude 
of  the  place. 

To  understand  this,  let  us  suppose  that  we  are  at  the  equator  ;  then 
the  polar  star  will  be  in  our  horizon,  being  90°  from  our  zenith,  or  the 
point  over  our  heads.  Now  suppose  we  travel  1°  on  a  meridian  line,  or 
directly  towards  the  north  pole,  then  the  polar  star  will  appear  elevated 
1°  above  our  horizon;  by  travelling  2°,  the  polar  star  will  appear  ele- 
vated 2°  ;  half  way  between  the  equator  and  the  pole,  our  distance  from 
the  equator  will  be  45°,  and  then  the  polar  star  will  appear  to  us  elevated 
45°,  and  so  on.  Thus  it  is  that  the  elevation  of  the  polar  star  gives  us 
the  latitude  of  the  place.  By  this  means  navigators  sailing  on  an  ex- 
panse of  ocean  can  find  the  latitude  of  the  place  where  they  are. 

MEASUREMENT    OF    THE    EARTH. 

29.  The  same  course  of   reasoning  will  show  how  a  degree  on  the 
earth's  surface  is  measured.     At  London,  the  elevation  of  the  polar  star 
is  about  51i°  ;  now  if  wre  travel  due  north  until  we  find  its  elevation  to 
be  52£°,  we  shall  have  travelled  over  1°,  or  the  360th  part  of  the  earth's 
circumference ;  and  if  this  distance  be  accurately  measured,  it  will  be 
found  to  be  about  69£  miles,  which  is  consequently 'the  length  of  a  de- 
gree.    The  whole  circumference  of  the  earth  will  therefore  be  about  360 
times  69i  miles,  or,  in  round  numbers,  25,000  miles. 

It  must,  however,  be  observed,  that  the  earth  is  not  an  exact  sphere,  for 
it  has  been  found  that  the  length  of  a  degree  measured  towards  the 
poles  is  greater  than  it  is  at  the  equator ;  thereby  showing  that  the  earth 
is  a  little  flattened  at  the  poles,  so  that  the  diameter  passing  through  the 
equator  is  about  26  miles  greater  than  the  diameter  passing  through  the 
poles. 

ANNUAL    MOTION    OF    THE    EARTH. CAUSE    OF    THE 

SEASONS. 

30.    Besides  the  spinning  motion  of  the  earth  upon  'its  axis, 
we  have  said  that  it  moves  round  the  sun  in  the  course  of  a 
32 


374          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

year,  in  a  path,  or  orbit,  which  is  nearly  circular.  This  an- 
nual motion,  combined  with  the  unchanging  direction,  or  par- 
allelism, of  the  earth's  axis,  is  the  cause  of  the  seasons. 

Let  the  small  globe  be  carried  round  a  candle  (covered  with  a  glass 
shade  about  the  same  size  as  the  globe)  at  the  same  time  that  it  is  kept 
spinning  upon  its  axis ;  then  we  shall  have  a  tolerably  correct  exhibition 
of  the  twofold  motion  of  the  earth,  viz.,  its  diurnal  or  daily  motion  on 
its  axis',  and  its  annual  motion  round  the  sun.  The  path  in  which  the 
globe  is  moved  will  represent  the  orbit  of  the  earth,  and  a  level  or  even 
surface  going  through  this  path  will  represent  the  plane  of  the  earth's 
orbit.  Again,  let  our  little  globe  be  carried  round  the  candle,  with  its 
axis  perpendicular  or  tipright  to  the  plane  cf  the  orbit ;  then  it  Avill  be 
seen  that  the  circle  on  the  globe  separating  the  light  and  shade  passes 
through  the  poles  throughout  the  whole  revolution  ;  this  position  of  the 
axis,  therefore,  will  not  account  for  the  changes  of  the  seasons. 

Let  the  globe  be  now  carried  round  the  candle  with  the  axis  constantly 
inclined  to  the  plane,  or  surface  of  the  table,  at  the  same  angle ;  thfcn,  in 
every  position  of  the  globe,  it  will  be  seen  that  the  axis  always  lies  in 
the  same  direction,  or  that  it  is  always  parallel  to  itself. 


Fig.  14.     Cause  of  the  Seasons. 

Let  the  globe  have  the  position  c  in  the  figure,  where  the  axis  is  in- 
clined towards  the  sun,  so  that  a  rod  extended  from  the  candle,  repre- 
senting the  sun,  shall  be  perpendicular  to  the  tropic  of  Cancer,  at  c ; 
then,  as  the  light  will  extend  over  90°  every  way  from  c,  the  circle  e  f 
which  separates  the  light  and  shade  will  pass  23£°  over  and  beyond  the 
north  pole ;  and  therefore,  during  the  revolution  of  the  globe  on  its  axis, 


ASTRONOMY.  375 

the  whole  of  the  north  frigid  zone  will  be  enlightened,  and  on  the 
contrary,  the  whole  of  the  south  frigid  zone  will  be  in  darkness.  In 
order  to  illustrate  this,  suppose  a  little  fly  were  placed  upon  the  arctic 
circle ;  then  throughout  a  whole  revolution  the  creature  will  not  have 
gone  without  the  light  of  the  candle ;  and  on  the  contrary,  let  the  crea- 
ture be  placed  upon  the  antarctic  circle  ;  then,  throughout  a  whole  revo- 
lution it  will  not  have  come  within  the  light  at  all.  It  will  also  be  seen 
that  all  places  in  the  northern  hemisphere  will  be  longer  in  the  circle  of 
light  than  in  the  circle  of  darkness;  and  on  the  contrary,  all  places  in 
the  southern  hemisphere  will  be  longer  in  the  circle  of  darkness  than  in 
the  circle  of  light ;  that  is,  in  the  former  hemisphere,  the  day,  as  in  our 
summer,  will  exceed  twelve  hours  ;  whilst  in  the  latter  hemisphere,  the 
day  will  be  less  than  twelve  hours.  Whereas,  exactly  on  the  equator, 
the  days  will  not  alter  in  their  length.  This  position  of  the  globe  cor- 
responds to  our  midsummer,  or  21st  of  June. 

Constantly  keeping  the  axis  pointing  in  the  same  direction,  let  the 
globe  be  brought  to  the  position  b  of  the  figure,  where  the  axis  neither 
incline^  to  the  sun  nor  from  the  sun ;  now  the  light  will  fall  perpendic- 
ularly on  the  equator ;  the  circle  separating  the  light  and  shade  will  pass 
through  the  poles,  and  therefore  the  days  and  nights  will  be  equal  all 
over  the  globe.  This  position  corresponds  to  our  autumnal  equinox,  the 
22d  of  September,  or  to  that  time  in  autumn  when  the  length  of  the 
night  equals  the  length  of  the  day.  Still  keeping  the  axis  pointing  in 
the  same  direction,  let  the  globe  be  now  brought  to  the  position  q,  where 
the  north  pole  inclines  away  from  the  sun.  Here  the  reverse  of  what  was 
observed  in  the  first  position  c  will  now  take  place.  The  sun  will  shine 
perpendicularly  over  the  tropic  of  Capricorn,  and  the  southern  hemi- 
sphere will  enjoy  more  of  the  sun's  light  and  heat  than  the  northern. 
This  position  corresponds  to  our  midwinter,  the  21st  of  December,  and 
then  our  days  will  be  at  their  shortest. 

Let  the  globe  now  be  brought  to  the  position  d  of  the  figure ;  then, 
here  again,  the  axis  neither  inclining  to  the  sun  nor  from  the  sun,  the 
days  and  nights  will  be  equal,  as  at  the  autumnal  equinox.  This  posi- 
tion corresponds  to  our  vernal  or  spring  equinox,  the  20th  of  March. 

When  the  globe  is  brought  to  the  position  c,  it  has  made  a  complete 
revolution  in  its  orbit,  and  the  period  corresponds  to  our  natural  year,  or 
365  days,  5  hours,  48  minutes,  and  51  seconds.  Particular  attention 
should  be  given  to  the  circumstance  that  the  axis  of  the  globe,  through- 
out the  whole  revolution,  has  maintained  its  parallelism. 

31.  While  the  earth  thus  performs  a  revolution  in  its  orbit,  the  sun 
•wall  appear  to  us  to  make  a  complete  revolution  in  the  heavens,  through 
the  different  constellations  in  the  zodiac  or  belt  of  stars.  Thus,  in  our 
midsummer,  the  sun  will  be  referred  to  the  sign  23,  or  constellation  of 


376         NATURAL   AND   EXPERIMENTAL   PHILOSOPHY. 

Cancer  ;  in  our  autumnal  equinox,  to  the  sign  of  Libra,  or  ^  ;  in  our 
midwinter,  to  the  sign  of  Capricornus,  or  ]ff ;  and  in  our  vernal  equinox, 
to  the  sign  of  Aries,  or  cp. 

32.  Thus  the  changes  of  the  seasons,  as  well  as  the  apparent  annual 
motion  of  the  sun,  are  perfectly  explained  by  supposing  the  earth  to 
move  round  the  sun.     But  why,  it  may  be  asked,  do  we,  in  opposition 
to  the  first  impression  of  our  senses,  believe  that  the  earth  moves,  instead 
of  the  sun  ?     Just  for  the  same  reason  that  we  infer  that  the  apparent 
diurnal  revolution  of  the  sun  round  the  earth  is  produced  by  the  actual 
rotation  of  the  earth  on  its  axis  in  every  twenty-four  hours. 

33.  The  distance  of  the  earth  from  the  sun  is  about  ninety-five  mil- 
lions of  miles.     In  order  to  form  some  conception  of  this  immense  dis- 
tance, let  us  suppose  a  body  to  move  from  the  earth  to  the  sun  with  the 
speed  of  one  of  our  railway  carriages,  (50  miles  per  hour ;)  then  it  would 
take  about  220  years  to  arrive  at  the  sun. 


THE  MOON. 

34.  The  diameter  of  the  moon  is  about  2000  miles,  or 
about  one  fourth  the  diameter  of  the  earth  ;  she  performs  a 
revolution  round  the  earth  in  27  days,  7  hours,  43  minutes,  in 
an  orbit  whose  radius  is  about  240,000  miles,  or  about  60 
times   the   earth's   radius.     The   moon  always  presents  the 
same  face  to  us ;  hence  it  follows  that  she  must  turn  round  on 
her  axis  in  the  same  time  that  she  revolves  round  the  earth. 

MOUNTAINS    AND    CAYITIES    ON   THE   MOON. 

35.  When  the  moon  is  viewed  through  a  telescope,  various 
spots,  of  different  degrees  of  brightness  and  depth  of  shade, 
are  observed  on  her  surface.     The  darkest  portions  are  caused 
by  deep  cavities  and  valleys  ;  those  of  a  lighter  shade  by  the 
shadows  of  high  mountains  ;  and  the  brightest  spots  are  the 
illuminated  tops  of  the  mountains,  which  look  like  the  craters 
of  volcanoes. 

The  heights  of  many  of  the  mountains  on  the  moon  have  been  calcu- 
lated'from  the  lengths  of  the  shadows  which  they  cast.  'The  loftiest  of 
them  are  about  two  miles  high.  The  moon  has  no  clouds,  nor  have  any 
decided  indications  of  an  atmosphere  been  observed.  It  therefore  seems 
improbable  that  living  beings,  such  as  we  are,  can  exist  there. 


ASTRONOMY.  377 

The  Earl  of  Rosse,  who  has  recently  completed  another  telescope,  the 
largest  ever  made,  alluded,  at  a  late  meeting  in  London,  to  its  effects. 
He  said  that,  with  respect  to  the  moon,  every  object  on  its  surface  of  100 
feet  in  height  was  now  distinctly  to  be  seen ;  and  he  had  no  doubt, 
under  very  favorable  circumstances,  it  would  be  so  with  objects  60  feet  in 
height.  On  its  surface  were  craters  of  extinct  volcanoes,  rocks,  and 
masses  of  stones,  almost  innumerable.  There  were  no-  signs  of  habitations 
such  as  ours  :  no  vestiges  of  architecture  remain  to  show  that  the  moon 
is  or  ever  was  inhabited  by  a  race  of  mortals  similar  to  ourselves.  It 
presented  no  appearance  which  could  lead  to  the  supposition  that  it  con-  • 
tained  any  thing  like  green  fields  and  the  lovely  verdure  of  this  beautiful 
world  of  ours.  There  was  no  water  visible  —  not  a  sea  or  a  river  :  all 
seemed  desolate. 


PERIODICAL    AND    SYNODICAL    MONTH. 

36.  Like  the  sun  and  planets,  the  moon,  in  consequence  of 
her  revolution  round  the  earth,  has  an  apparent  motion  from 
west  to  east  among  the  stars  of  the  zodiac.  Her  motion 
among  the  stars  is  so  rapid  that  it  may  be  readily  perceived 
by  a  few  hours'  observation  on  any  moonlight  night.  As  al- 
ready stated,  she  makes  a  complete  revolution  in  the  heavens 
in  27  days,  7  .hours,  43  minutes  ;  that  is  to  say,  she  takes  this 
time  in  passing  from  a  star  to  returning  to  the  same  star  again : 
this  is  called  her  periodical  month  ;  but  the  time  from  new 
moon  to  new  moon  again  is  rather  longer  than  this,  in  conse- 
quence of  the  motion  of  the  earth  in  its  orbit.  The  time  be- 
tweeri  every  new  moon  is  29  days,  12  hours,  44  minutes  :  this 
is  called  the  synodical  month. 

Let  S  (Fig.  15)  represent  the  sun  ;  E  the  earth ;  A  B  a  part  of  its  orbit ; 
M  C  the  orbit  of  the  moon  round  the  earth ;  M  her  position  at  new  moon, 
which  is  in  a  line  drawn  from  the  earth  to  the  sun.  Now,  if  the  earth 
had  no  motion,  the. moon  would  move  round  in  her  orbit  and  return  to 
the  position  M  in  a  periodic  month  ;  but  while  the  moon  is  describing  a 
revolution,  the  earth  will  have  moved  over  about  the  twelfth  part  of  its 
orbit,  suppose  from  E  to  e.  The  moon  will  then  be  at  n,  where  c  n  is 
drawn  parallel  to  E  M,  and  she  must  therefore  move  over  an  additional 
portion  n  m  of  her  orbit  before  she  comes  again  in  the  line  c  S  joining  the 
earth  and  the  sun.  This  additional  portion,  being  about  the  twelfth  part 
of  her  whole  orbit,  occasions  the  time  of  the  snodical  revolution  to 


378          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

exceed  the  periodical  by  rather  more  than  two  days.     This  is  well  illus- 
trated by  the  relative  motions  of  the  hour  and  minute  hands  of  a  watch : 


Fig.  15.    Periodical  and  Synodical  Month. 

at  12  o'clock  the  hands  are  together,  but  before  they  can  come  together 
again  the  minute  hand  must  move  over  a  whole  revolution  and  rather 
more  than  the  twelfth  part  of  another  one. 


THE   MOON'S    PHASES. 

37.  The  sun  always  enlightens*  one  half  of  the  moon ;  but 
as  her  enlightened  hemisphere  is  always  directed  towards  the 
sun,  she  presents  different  phases  of  illumination  to  us  as  she 
moves  in  her  orbit.  Sometimes  we  see  the  whole  of  her  en- 
lightened disk,  sometimes  only  a  small  portion  of  it,  and  at 
other  times  none  at  all. 

Let  E  represent  the  earth ;  (see  Fig.  16  ;)  S  the  sun  ;  and  a,  b,  c,  d,  e, 
ft  g,  h  the  moon  in  different  parts  of  her  orbit,  having  her  enlightened 
hemisphere  always  turned  towards  the  sun.  The  little  circles  outside 
of  the  line  representing  the  moon's  orbit  show  the  moon's  faces  at  the 
respective  positions,  as  seen  by  an  observer  on  the  earth. 

When  the  moon  is  at  a,  in  a  line  with  the  earth  and  the  sun,  the  dark 
face  of  the  moon  is  turned  towards  the  earth  ;  the  moon  is  then  at  her 
change,  or  she  is  called  new  moon.  She  is  also  at  this  time  in  conjunction 
with  the  sun. 

At  b  a  small  portion  of  her  enlightened  hemisphere  is  turned  towards 
the  earth,  and  she  then  appears  horned. 

At  c  one  half  of  her  enlightened  hemisphere  is  turned  towards  the 


ASTRONOMY. 


379 


earth,  and  she  then  appears  as  half  moon.    This  takes  place  at  the  end 
of  her  first  quarter,  or  at  her  quadratures. 

At  d  about  three  quarters  of  her  enlightened  hemisphere  is  visible  to 
us,  and  she  is  then  said  to  be  gibbous. 


O 


Fig.  16.    The  Moon's  Phases. 


At  e,  when  she  has  completed  one  half  of  her  revolution,  the  whole 
of  her  enlightened  hemisphere  is  visible  to  us,  and  she  is  then/iJJ  moon. 
In  this  position  she  is  said  to  be  in  opposition  to  the  sun.  If  the  plane 
of  the  moon's  orbit  had  exactly  coincided  with  that  of  the  earth's,  she 
would  have  been  invisible  to  us  at  this  period,  for,  in  this  case,  the  earth 
would  have  obstructed  the  sun's  light ;  but  it  so  happens,  that  she  is 
mostly  either  above  or  below  the  line  connecting  the  earth  and  the  sun, 
and  hence  it  is  that  we  usually  see  the  whole  of  her  enlightened  face. 
This  will  be  better  understood  when  we  come  to  consider  the  subject  of 
eclipses. 

At  /  she  is  gibbous,  at  g  half  moon,  at  h  horned,  and  at  a  she  again 
becomes  invisible. 


ECLIPSES. 

38.  An  eclipse  of  the  sun  is  called  a  solar  eclipse,  and  that 
of  the  moon  a  lunar  eclipse.  When  the  moon  comes  between 
the  earth  and  the  sun,  his  light  is  obstructed,  and  an  eclipse 
of  the  sun  is  produced ;  and  an  eclipse  of  the  moon  takes 
place  when  the  earth  is  between  the  sun  and  the  moon.  Hence 
it  is  that  eclipses  of  the  moon  can  only  occur  at  her  fully  or 


380  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

when  she  is  in  opposition,  and  eclipses  of  the  sun  at  her  change, 
or  when  she  is  in  conjunction  ;  moreover,  the  three  bodies  must 
be  in,  or  nearly  in,  the  same  straight  line. 

Now,  if  the  moon's  orbit  were  in  the  same  plane  as  the  ecliptic  or  path 
of  the  earth,  then  the  sun  would  be  eclipsed  at  every  new  moon,  and  the 
moon  would  be  eclipsed  at  every  full  moon.  But  as  her  orbit  is  a  little 
inclined  to  the  earth's,  she  is  mostly  either  above  the  ecliptic  or  below  it 
when  she  is  in  opposition  and  conjunction.  The  points  where  the  moon's 
orbit  cuts  the  plane  of  the  ecliptic  are  called  the  nodes  y  hence  it  follows 
that  eclipses  can  only  take  place  when  the  moon  happens  to  be  in  or 
near  one  of  the  nodes  at  the  moment  she  is  in  opposition  or  conjunction. 
In  the  course  of  a  year  there  may  be  seven  eclipses  of  the  sun  and 
moon  —  five  of  the  sun  and  two  of  the  moon,  or  four  of  the  sun  and 
three  of  the  moon.  Lunar  and  solar  eclipses  differ  very  much  from 
each  other  in  certain  respects  :  a  lunar  eclipse  may  be  seen  at  the  same 
time  by  all  persons  to  whom  the  moon  is  visible,  whereas  a  solar  eclipse 
may  be  seen  by  one  person  and  not  by  another  ;  again',  an  eclipse  of  the 
sun  can  never  last  more  than  eight  minutes,  whereas  an  eclipse  of  the 
moon  frequently  continues  for  more  than  two  hours. 

39.  Eclipse  of  the  moon.  —  If  the  whole  disk  or  face  of 
the  moon  is  immersed  in  the  shadow  cast  by  the  earth,  then 
the  eclipse  is  said  to  be  total ;  and  the  eclipse  is  said  to  be 
partial  when  only  a  part  of  the  disk  is  obscured. 


Fig.  17.     Total  Eclipse  of  the  Moon. 

In  Fig.  17  a  total  eclipse  of  the  moon  is  shown  ;  S  represents  the  sun ; 
E  e  the  earth  ;  A  B  the  moon's  orbit  round  the  earth  ;  E  e  V  the  conical 
shadow  cast  by  the  earth  ;  M  the  dark  body  of  the  moon  totally  im- 
mersed in  this  shadow. 

It  is  always  observed  that  the  edge  of  the  earth's  shadow  on  the  face 
of  the  moon  is  circular ;  now,  this  proves  that  the  earth  is  a  globe,  for  no 
body  but  a  globe  will  always  cast  a  circular  shadow.  Take  an  orange 
and  hold  it  on  a  level  with  the  flame  of  a  candle ;  observe  the  shadow 
which  is  cast  upon  a  sheet  of  paper  held  at  different  distances  from  the 
orange. 


ASTRONOMY.  381 

40.  Eclipse  of  the  sun.  —  A  total  eclipse  of  the  sun  takes 
place  at  that  part  of  the  earth's  surface  which  is  immersed 
in  the  moon's  shadow. 

Fig.  18  represents  a  total  eclipse  of  the  sun ;  where  S  represents  the 
sun ;  E  e  the  earth ;  A  B  the  moon's  orbit ;  M  the  moon  exactly  in  a 


Fig.  18.    Total  Eclipse  of  the  Sun. 


line  between  the  sun  and  the  earth ;  en  ao  the  moon's  shadow  cast 
upon  a  small  portion  of  the  earth  at  a  o :  this  dark  shadow  is  called  the 
umbra.  The  sun  will  appear  totally  eclipsed  to  persons  living  within  a  o  ; 
but  to  persons  living  without  this  portion,  that  is,  between  a  o  and  E  e, 
the  sun  will  be  visible.  Between  a  o  and  b  r  the  sun  will  be  partially 
obscured :  this  portion  of  the  shadow  is  called  the  penumbra,  because  it 
is  not  so  dark  as  the  umbra,  or  the  portion  in  the  full  shadow. 

Within  the  umbra,  therefore,  a  total  eclipse  takes  place ; 
whereas  within  the  penumbra  the  sun  is  only  partially  eclipsed. 

41.  Annular  eclipse.  —  If  the  conical  shadow  of  the  moon 
does  not  reach  the  earth,  then  an  annular  eclipse  will  take 


Fig.  19.    Annular  Eclipse. 

• 

place  to  all  persons  immediately  below  the  vertex  of  the 
moon's  shadow ;  that  is,  the  moon  will  appear  like  a  black  spot 
upon  the  sun,  surrounded  by  a  ring  of  light. 

Here  the  vertex  of  the  moon's  conical  shadow  does  not  reach  the  earth 


382  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

at  a  ;  so  that  a  spectator  at  a  will  see  the  moon  like  a  dark  spot  nearly 
covering  the  sun's  disk. 


THE   SUN  AND  PLANETS. 

42.  Having  described  the  motions  of  the  earth  and  the 
moon,  we  shall  now  treat  of  the  sun,  with  the  other  bodies 
composing  the  solar  system. 

43.  The  planets  are  opaque  bodies  ;  that  is  to  say,  they  do 
not  emit  any  light  of  their  own,  but  merely  shine  with  the 
light  borrowed  from  the  sun.     This  is  proved  by  means  of  the 
telescope,  which  shows  that  they  present  faces  similar  to  the 
moon's,  having  their  enlightened  sides  always  turned  towards 
the  sun. 

44.  The  planets  are  divided  into  inferior  and  superior : 
those  which  revolve  within  the  earth's  orbit  are  called  inferior 
planets,  and  those  which  revolve  without  it  are  called  superior 
planets.     Thus  Mercury  and  Venus  are  inferior  planets,  and 
Mars,  Jupiter,  Saturn,  Uranus,  and  Neptune,  together  with 
the  Asteroids,  are  superior  planets.-    (See  Fig.  3.) 


APPARENT    MOTIONS    AND    APPEARANCES    OF    THE    PLANETS 
EXPLAINED. 

45.  Viewed  from  the  sun,  as  the  great  centre  of  the  solar 
system,  the  planets  would  appear  to  move  round  him  in  regu- 
lar order  and  progression.  But  the  case  is  very  different 
when  we  view  their  motions  from  the  earth,  which  also  moves 
round  the  sun ;  at  one  time  they  appear  to  have  a  progressive 
or  direct  motion,  that  is,  from  west  to  east;  then  they  ap- 
pear stationary,  or  without  any  apparent  motion  ;  and  at  other 
times  they  appear  to  have  a  retrograde  motion,  that  is,  from 
east  to  west.  They  are  sometimes  in  conjunction  with  the 
sun.  and  then  they  are  generally  lost  in  his  superior  light ; 
and  some  of  them  (the  superior  planets)  appear  in  opposition 
to  the  sun,  that  is,  in  the  opposite  point  of  the  heavens. 


ASTRONOMY. 


383 


In  order  to  form  a  familiar  idea  of  these  motions,  conceive  yourself 
placed  in  the  centre  of  a  horse  ring  ;  the  horse,  as  he  moves  round  you, 
will  appear  to  move  in  a  regular  and  progressive  manner  :  now,  conceive 
yourself  to  be  placed  without  the  ring ;  then  the  motion  of  the  horse  ap- 
pears no  longer  regular  :  at  one  time  he  appears  to  move  say  from  right 
to  left,  then  for  a  moment  he  appears  as  if  he  were  stationary,  and  after- 
wards he  appears  to  move  from  left  to  right,  and  in  two  points  of  his 
path  he  appears  in  conjunction,  or,  as  it  were,  in  the  same  place  with 
the  man  in  the  centre  of  the  ring.  These  apparent  motions  of  the  horse 
give  a  true  representation  of  the  apparent  motions  of  the  two  inferior 
planets,  Mercury  and  Venus. 

46.  Opposition  and  conjunction  of  the  planets.  —  That 
Mercury  and  Venus  are  inferior  planets  is  proved  by  their 
crossing  the  sun's  disk  like  a  black  spot,  thereby  showing  that 
they  must  revolve  between  us  and  the  sun ;  whereas  Mars 
and  the  other  superior  planets  never  do  so.  Moreover,  Mer- 
cury and  Venus  never  appear  in  opposiXion ;  whereas  Mars 
and  the  other  superior  planets  appear  in  opposition  as  well  as 
in  conjunction. 

In  Fig.  20  let  S  represent  the  sun ;  E  the  earth ;  V  an  inferior  planet ; 
and  M  a  superior  one.  At  m  and  v  both  planets  appear  in  conjunction 


Fig.  20.     Conjunction  and  Opposition. 


to  a  spectator  on  the  earth,  but  at  M  and  V  the  superior  planet  M  is  in 
opposition,  while  the  inferior  planet  V  is  in  conjunction;  and  at  this 


384 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


position  it  will  sometimes  appear  like  a  black  spot  crossing  the  sun's 
disk  :  this  is  called  the  transit  of  Venus,  or  Mercury,  as  the  case  may 
be :  thus,  while  the  superior  planets  never  cross  the  sun's  disk,  the  infe- 
rior ones  never  appear  in  opposition. 

47.  Apparent  motions  of  Venus.  —  We  shall  now  illustrate  the  cause 
of  the  apparent  motions  and  phases  of  the  planets  by  a  reference  to  the 
planet  Venus. 


Fig.  21.     Apparent  Motions  of  Venus. 

In  Fig.  21,  let  S  represent  the  sun ;  E  the  earth ;  abcdefghihe 
different  positions  of  Venus  in  her  orbit;  E  a  &  and  E  g /tan gent  lines 
drawn  from  the  earth  to  the  orbit  of  Venus.  From  a  to  b,  the  planet 
Venus  appears  stationary,  that  is,  for  a  .time  she  neither  appears  to 
move  towards  the  west  nor  towards  the  east ;  in  this  position  she  has 
attained  her  greatest  westerly  distance,  or  elongation,  from  the  sun. 
From  b  to  f  her  motion  is  direct,  that  is,  she  appears  to  move  amongst 
the  stars  from  west  to  east!  From/  to  g  she  is  again  stationary ;  and 
from  g  to  a  her  motion  is  retrograde,  that  is,  she  appears  to  move  from 
east  to  west.  At  h,  in  a  line  with  the  earth  and  sun,  a  transit  takes 
place.  Thus,  in  making  an  apparent  revolution  from  a  round  the  sun, 
she  is  first  stationary,  then  she  has  a  direct  motion,  next  stationary,  and, 
finally,  she  has  a  retrograde  motion. 

48.  Phases  of  Venus.  —  Between  h  and  a,  (see  Fig.  21,)  her  enlight- 
ened hemisphere  appears  to  us  like  a  horned  moon ;  at  a  and  b  she  pre- 
sents the  appearance  of  a  haJfmoon;  at  c^ gibbous  ;  and  at  d,futt  moon ; 


ASTRONOMY. 


385 


and  so  on.     It  is  plain  that  if  Venus  had  shone  with  her  own  light,  she 
would  always  have  appeared  perfectly  round  to  us. 

49.  Morning  and  evening  star.  —  When  Venus  appears  to  the  west 
of  the  sun,  that  is,  from  d  to  h,  (see  Fig.  21,)  she  is  the  evening  star, 
for  then  she  shines  in  the  western  sky  at  sunset ;  and  on  the  contrary, 
when  she  appears  to  the  east  of  the  sun,  that  is,  from  h  to  d,  she  shines 
in  the  eastern  sky  before  sunrise. 


COMPARATIVE    SIZE    AND  APPEARANCE    OF    THE    PLANETS. 

50.   The  following  diagram  exhibits  the  comparative  size  and  appear- 
ance of  the  principal  planets  in  the  solar  system. 


Fig.  22. 

Jupiter  is  the  largest  of  all  the  planets;  his  diameter  is  about  11 
times  the  diameter  of  the  earth ;  Saturn,  Neptune,  and  Uranus  are  next 
in  order  of  magnitude ;  the  Earth  and  Venus  are  about  the  same  size ; 
the  diameter  of  Mars  is  only  about  one  half  the  diameter  of  the  earth ; 
and  Mercury  is  about  one  third  smaller  than  Mars.  The  Asteroids 
(which  could  not  be  shown  in  this  diagram)  are  very  small  bodies,  the 
largest  of  them  not  being  more  than  250  miles  in  diameter.  The  diam- 
eter of  the  sun'  is  about  110  times  the  diameter  of  the  earth,  and  his 
entire  mass  is  vastly  greater  than  that  of  all  the  planets  put  together. 
Constructed  on  the  scale  of  the  accompanying  diagram,  he  would  have 
been  represented  by  a  globe  of  about  a  foot  in  diameter. 
33 


386 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


TABULAR    VIEW    OF    THE    SOLAR    SYSTEM. 


NAMES. 

Diameter  in 

Miles. 

Distance  from 
the    Sun    in 

Miles. 

Time  of  Rota- 
tion on  Axis. 

Annual  Revo- 
lution round 
the    Sun   in 
Days. 

Sun 

SS2000 

H.      M. 

607  48? 

3140 

37  mill'ns 

24    5 

87-969 

Venus  

7800 

69      " 

23  21 

224-700 

Earth 

7926 

95      " 

24    0 

365-256 

4100 

144      " 

24  37 

686-979 

f  Flora,  

209       " 

1193-249 

Vesta 

250' 

225 

1325-147 

Iris 

226 

1341-636 

g 

Metis 

227       ' 

1345-850 

3 

Hebe 

230     ' 

1379-994 

®    1 

244 

1511-095 

"Jo 

79? 

250 

27     0? 

1594-296 

•*5 

Ceres 

160? 

260 

1682-125 

Pallas'      

261 

1686-510 

Irene.* 

87000 

490 

•9  56 

4332-584 

Saturn,  

79160 

900 

10  29 

10759-219 

34500 

1800 

9  30? 

30686-820 

41500 

2850 

60126-710 

THE    SUN. 

51.  This  stupendous  globe,  nearly  a  million  and  a  half 
times  the  bulk  of  our  earth,  is  the  great  source  of  light  and 
heat  to  all  the  planets,  and  by  the  attraction  which  he  exerts 
retains  them  in  their  orbits.     The  telescope  shows  that  there 
are  dark  spots  upon  his  surface,  and  by  observing  them,  as- 
tronomers have  ascertained  that  he  revolves  on  his  axis  every 
25  days,  in  the  same  direction  as  the  planets  move  round  him, 
that  is,  from*  west  to  east. 

MERCURY. 

52.  This  little  planet  moves  round  the  sun  in  about  88 
days,  at  the  distance  of  about  37,000,000  of  miles,  and  re- 
volves on  his  axis  in  24  hours  5  minutes.     The  length  of  his 
day  will,  therefore,  be  rather  more  than  ours,  and  the  dura- 


*  Discovered  by  Mr.  Hind,  May  19, 1851. 
been  very  recently  discovered. 


Three  additional  asteroids  have 


ASTRONOMY.  387 

tion  of  his  year  about  one  fourth  that  of  our  year.     The  appa- 
rent motions,  &c.,  of  this  planet  are  similar  to  those  of  Venus. 

VENUS. 

53.  Of  all  the  stars,  this  is  the  brightest  and  most  beauti- 
ful.    Her  distance  from  the  sun  is  about  three  fourths  of  the 
earth's  distance,  and  hence  she  receives  nearly  double  the  light 
and  heat  from  the  sun.*     She  completes  her  revolution  round 
the  sun  in  about  225   days,  and  performs  a  rotation  in  23 
hours  21  minutes,  on  an  axis  inclined  to  the  plane  of  her  orbit 
at  an  angle  of  15°.     The  length  of  her  day  is,  therefore, 
nearly  the  same  as  ours,  and  the  inclination  of  her  axis  shows 
that  she  has  seasons  similar  to  ours.     She  is  surrounded  by  a 
large  atmosphere,  and  from  the  irregularities  observed  on  the 
edge  of  her  crescent,  it  has  been-  inferred  that  she  has  enor- 
mous mountains  upon  her  surface,  probably  much  larger  than 
any  on  our  earth. 

MARS.  . 

54.  This  small  planet  is  about  li  times  the  earth's  distance 
from  the  sun ;  he  takes  about  two  of  our  years  in  revolving 
round  the  sun;  and  the  length  of  his  days  is  about  the  same 
as  ours.     The  inclination  of  his  axis  to  the  plane  of  his  orbit 
shows  that  he  has  seasons  similar  to  those  which  take  place  on 
the  earth.     He  is  surrounded  by  an  atmosphere,  and  the  out- 
line of  continents  and  seas  may  be  distinctly  traced  by  means 
of  a  telescope.     The  red,  fiery  color  of  his  light  is  supposed 
to  be  produced  by  the  ochrey  tinge  of  his  soil,  like  that  which 
red  sandstone  might  produce.     Bright  white  spots  are  seen 
about  the  poles,  which  are  no  doubt  occasioned  by  the  reflec- 
tion of  the  sun's  light  from  the  polar  snows  and  ice  upon  the 
planet ;  for  it  is  observed  that  as  each  pole  is  turned  towards 
the  sun,  the  bright  spots  about  it  become  less,  owing  to  the 
melting  of  the  snow  by  the  sun's  heat. 

*  The  light  and  heat  derived  from  a  luminous  body  varies  inversely  as  the 
squares  of  the  distance :  thus,  taking  the  earth's  distance  from  the  sun  as 
unity,  we  have  heat  of  the  earth :  heat  of  Venus  : :  (I)2  : 12  :  :  9  :  16. 


388          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 
THE    ASTEROIDS. 

55.  These  bodies  revolve  round  the  sun,  in  orbits  variously 
inclined  to  the  ecliptic,  between  the  orbits  of  Mars  and  Jupi- 
ter. They  are  so  very  small  that  their  diameters  have  not 
yet  been  accurately  determined.  Some  of  them  have  very 
extensive  atmospheres.  They  have  all  been  discovered  with- 
in the  present  century,  and  six  of  them  have  been  discovered 
within  the  last  six  years. 


JUPITER. 

56.  This  is  the  largest  of  the  pianets.     He  takes  about 
twelve  years  to  complete  his  revolution  round  the  sun,  and 
turns  upon  his  axis  in  about  ten  hours.     This  rapid  rotation 
has  caused  him  to  be  much  flattened  at  the  polesi 

The  disk  of  Jupiter  is  always  found  to  be  crossed  with 
dark  parallel  bands  or  belts  with  spots,  as  shown  in  Fig.  22. 
Although  these  behs  vary  both  in  breadth  and  situation,  yet 
they  always  run  parallel  to  the  equator  of  the  planet ;  this 
appearance  of  the  planet,  no  doubt,  depends  upon  its  atmos- 
phere. 

This  magnificent  planet  has  four  moons,  which  constantly 
revolve  about  him  from  west  to  east,  and  accompany  him  in 
his  path  round  the  sun.  Thus  the  satellites  of  Jupiter  con- 
stitute a  miniature  system,  to  which  their  primary  is  the  cen- 
tre, in  all  respects  similar  to  the  solar  system,  of  which  their 
central  body  itself  is  only  a  member. 

Three  of  Jupiter's  satellites  are  totally  eclipsed  at  every 
revolution,  by  the  great  shadow  which  he  casts  from  the  sun. 
These  eclipses  are  of  great  use  in  finding  the  longitude  of 
places  upon  the  earth. 

57.  Velocity  of  light.  —  The  eclipses  of  Jupiter's   satel- 
lites have  enabled  astronomers  to  determine  the  velocity  of 
light.     "When  Jupiter  is  in  opposition  we  are  much  nearer  to 
him  than  when  he  is  in  conjunction ;  owing  to  this  difference 


ASTRONOMY.  389 

of  distance  we  see  the  eclipses  of  his  satellites  16i  minutes 
sooner  in  the  one  position  than  we  do  in  the  other. 

Let  S  represent  the  sun,  (see  Fig.  23  ;)  J  Jupiter ;  M  the  satellite 
eclipsed  by  the  great  conical  shadow  of  the  planet ;  E  the  position  of  the 


Fig.  23.    Eclipse  of  Jupiter's  Satellites. 

earth  when  Jupiter  is  in  or  nearly  in  opposition  ;  and  e  the  position  of  the 
earth  when  he  is  in  conjunction ;  then  the  distance  between  E  e  is  equal 
to,  or  nearly  equal  to,  the  diameter  of  the  earth's  orbit.  Now,  the 
eclipse  seen  from  E  takes  place  8£  minutes  before  the  calculated  time, 
whereas  when  it  is  seen  from  e  it  takes  place  8|  minutes  later  than  the 
calculated  or  true  time;  consequently  the  light  takes  16£  minutes  to 
travel  from  E  to  e ;  that  is,  light  takes  16^  minutes  in  traversing  the 
diameter  of  the  earth's  orbit. 

SATURN. 

58.  Saturn's  year  is  29£  times  the  length  of  our  year,  and 
the  length  of  his  day  is  about  10£  hours.  His  distance  from 
the  sun  is  about  9i  times  that  of  the  earth.  The  diameter 
at  his  equator  is  about  T^  greater  than  the  diameter  at  his 
poles.  Like  the  earth,  his  axis  is  inclined  to  the  plane  of  his 
orbit,  and  therefore  he  must  have  seasons.  Saturn  has  eight 
satellites,  seven  of  which  had  been  known  for  sixty  years  be- 
fore the  eighth  satellite  was  discovered.  He  is  distinguished 
by  having  a  thin  broad  ring  surrounding  his  equator,  as 
shown  in  Fig.  22.  This  ring  is  concluded  to  be  opaque,  be- 
cause it  casts  a  shadow  on  the  surface  of  the  planet;  it  is  sep- 
arated by  different  intervals,  so  that  it  is  really  a  series  of 
rings  concentric  with  the  planet ;  its  whole  breadth  is  27,000 
miles,  and  its  thickness  does  not  exceed  100  miles.  The  space 
between  the  inner  side  of  the  ring  and  the  planet  is  19,000 
33* 


390  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

miles.  The  different  parts  of  the  ring  revolve  round  Saturn 
in  periods  depending  on  their  respective  distances  from  him  ; 
the  outermost  ring  revolves  in  about  10i  hours.  Saturn  has 
dark  belts  like  Jupiter,  but  rather  broader  and  less  strongly 
marked ;  the  cause  of  these  belts  is  no  doubt  atmospheric,  as 
in  the  case  of  the  belts  of  Jupiter. 

URANUS. 

59.  This  planet  completes  his  revolution  round  the  sun  in 
rather  more  than  eighty-four  years ;  his  mean  distance  from 
the  sun  is  about  nineteen  times  that  of  the  earth.     The  dis- 
coverer of  Uranus,  Sir  W.  Herschel,  believed  that  this  planet 
had  six  moons ;  but  only  two  have  been  observed  by  other 
astronomers.     The  motion  of  these  satellites,  round  their  pri- 
mary is  from  east  to  west,  which  is  an  exception  to  the  law 
observed  by  the  satellites  of  Jupiter,  Saturn,  and  the  Earth. 

NEPTUNE. 

60.  Neptune,  the  most  remote  planet  at  present  known  in 
the  solar  system,  completes  his  revolution  round  the  sun  in 
about  166  years,  at  about  thirty  times  the  distance  of  the 
earth  from  the  sun.     One  satellite  has  already  been  observed, 
revolving  round  the  planet  at  the  distance  of  about  twelve  of 
its  radii.     This  planet  was  discovered  in  1846,  and  its  exist- 
ence was  determined  by  calculations,  based  upon  the  law  of 
gravitation,  before  it  had  been  recognized  as  a  planetary  body 
by  observation.     This  may  be  regarded  as  one  of  the  greatest 
achievements  of  mathematical  science. 

COMETS. 

61.  Upwards  of  130  comets  have  been  observed  at  different 
times,  but  only  three  have  been  identified  as  having  been  seen 
before.     The  comet  which  was  seen  in  1835,  called  Halley's 
comet,  revolves  round  the  sun  in  about  seventy-six  years. 


ASTRONOMY.  391 

Their  orbits  are  ellipses  or  ovals,  so  very  flat  or  eccentric, 
that  the  comets  are  invisible  to  us  for  the  greater  part  of  their 
revolutions  round  the  sun. 

Comets  are  not  solid  like  the  planets ;  they  merely  consist 
of  a  mass  of  vapor,  the  central  portion  of  which  is  called  the 
nucleus,  or  head,  being  more  dense  than  the  rest.  Sometimes 
this  vapor  extends  to  a  great  distance  in  the  form  of  a  tail, 
which  is  always  in  a  direction  contrary  to  the  sun. 

THE    PLANETS    MOVE    IN    ELLIPSES. 

62.  The  true  path  of  the  planets  round  the  sun  are  ellipses  or  ovals, 
differing  in  general  but  little  from  circles,  of  which  the  sun  occupies  what 
is  called  the  focus. 

Thus,  in  Tig.  24,  E  represents  the  earth ;  E  D  A  its  elliptical  orbit 
round  the  sun ;  S  the  sun  in  the  focus  of  the  ellipse. 


Fig.  24.     Elliptical  Orbit. 

Kepler  discovered  the  elliptical  motion  of  the  planets,  with  other  im- 
portant facts,  by  observation,  and  Newton  showed,  by  mathematical  anal- 
ysis, that  this  peculiar  form  of  their  orbits  depends  upon  a  certain  law  of 
the  attractive  force  residing  in  the  sun,  called  the  law  of  gravitation. 

When  the  earth  is  nearest  the  sun,  as  at  p,  it  is  said  to  be 
in  its  perihelion  ;  and  when  the  earth  is  farthest  from  the  sun, 
as  at  a,  it  is  said  to  be  in  its  aphelion.  The  motion  of  the 
earth  in  its  orbit  is  quickest  when  it  is  nearest  the  sun,  or  in 
its  perihelion,  and  slowest  when  it  is  farthest  from  the  sun,  or 
in  its  aphelion.  Hence  it  is  that  the  time  between  our  vernal 
and  autumnal  equinoxes  is-  about  eight  days  longer  than  the 
time  between  our  autumnal  and  vernal  equinoxes ;  thereby 
causing  the  summer  in  the  northern  hemisphere  to  be  a  little 


392 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


longer  than  the  winter.  The  earth  is  about  three  millions  of 
miles  nearer  to  the  sun  in  winter  than  it  is  in  summer.  If 
this  be  the  case,  it  may  be  asked,  Why  is  our  summer  so  much 
warmer  than  our  winter?  If  we  are  nearer  to  the  sun  in 
winter  than  we  are  in  summer,  why  should  it  not  be  warmer 
in  winter,  rather  than  colder?  It  is  quite  true  that  this 
would  be  the  case,  were  it  not  for  other  causes,  which  far  more 
than  counterbalance  the  very  small  deficiency  of  temperature 
arising  from  this  difference  of  distance  from  the  sun.  These 
causes  have  been  briefly  and  incidentally  explained  in  Art.  30, 
but  it  may  be  instructive  to  bring  them  here  before  the  stu- 
dent in  a  distinct  form. 

Heat  of  Summer. 

The  days  in  our  summer  months  being  very  much  longer 
than  they  are  in  our  winter  months,  we  must  manifestly  re- 
ceive much  more  heat  from  the  sun  during  the  former  period 
than  we  do  during  the  latter. 

In  our  summer  the  sun  rises  to  a  much  greater  height  above 
the  horizon  than  he  does  during  our  winter,  and  consequently 
he  not  only  continues  longer  above  the  horizon,  but  his  rays, 
coming  more  perpendicularly,  strike  in  greater  numbers  upon 
any  given  portion  of  the  earth's  surface. 

Let  A  B  represent  a  portion  of  the  earth's  surface,  upon  which  the  rays 


Fig.  25.    Heat  of  Summer. 


ASTRONOMY.  393 

of  the  sun,  A  B  G  E,  fall  perpendicularly ;  and  let  A  C  be  an  equal  por- 
tion of  the  earth's  surface,  upon  -which  the  rays  of  the  sun,  A  C  F  E,  fall 
obliquely,  or  in  a  slanting  direction.  Now,  although  the  surfaces  A  B 
and  A  C  are  equal,  yet  it  is  plain  that  a  much  greater  number  of  rays 
must  fall  on  A  B  than  upon  AC:  the  rays  of  light  and  heat  falling 
upon  A  B  are  included  by  the  space  A  B  G  E,  whereas  those  which  fall 
on  A  C  are  included  by  the  space  A  D  F  E  :  in  fact  the  heat  which 
falls  upon  the  small  portion  A  D  is  spread  out  over  A  C. 

GRAVITATION. 

63.  When  a  body  moves  in  a  curved  line,  such  as  the  path 
of  the  earth  round  the  sun,  it  must  be  under  the  action  of  two 
forces,  one  an  impulsive  force,  or  force  of  projection,  the  other 
a  constantly  acting  force,  such  as  the  attraction  of  gravitation. 

We  have  a  familiar  instance  of  this  when  a  stone  is  projected  oblique- 
ly upwards  from  the  top  of  a  high  tower  ;  the  stone  moves  in  a  curve, 
called  a  parabola,  in  consequence  of  the  motion  of  projection  and  the  at- 
traction of  the  earth.  Now,  as  we  increase  the  force  of  projection,  the 
stone  will  be  longer  before  it  reaches  the  earth's  surface  ;  indeed,  it  is  not 
difficult  to  conceive  the  force  of  projection  to  become  so  great  that  the 
stone  shall  not  return  to  the  earth's  surface  at  all,  but  shall  move  round 
the  earth  like  a  little  satellite  similar  to  the  moon. 

The  earth  and  all  the  other  planets  had  at  first  a  motion  of  projection 
given  to  them  ;  and  this  motion  would  have  carried  them  away  into  in- 
finite space,  had  it  not  been  for  the  sun's  attraction.  If  the  attractive 
force  of  the  sun  were  to  cease,  the  earth  at  E  (see  Fig.  24)  would  fly  off 
from  its  orbit  in  the  tangent  line  E  K. ;  and  on  the  contrary,  if  the  motion 
of  projection  were  stopped,  the  earth  would  be  drawn  in  towards  the 
sun  ;  but  the  two  forces  of  projection  and  gravitation  are  so  nicely  ad- 
justed, that  the  earth  continually  moves  round  its  great  centre  of  attrac- 
tion, in  an  elliptical  orbit,  constantly  returning  at  every  revolution  (at 
least  virtually)  to  the  point  from  which  it  started.  This  law  of  gravita- 
tion,* which  holds  true  with  respect  to  the  sun  and  the  planets,  also 
holds  true  with  respect  to  the  motion  of  the  satellites  round  their  respec- 
tive primaries. 

*  According  to  the  law  of  gravitation,  (1.)  All  bodies  attract  one  an- 
other with  forces  proportional  to  the  masses  of  matter  which  they  contain ; 
(2.)  The  force  of  attraction  decreases  as  the  squares  of  the  distances 
increase. 


394  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


ATMOSPHERIC  REFRACTION. 

G4.  The  atmosphere  which  surrounds  the  earth  is  of  varia- 
ble density ;  that  is,  the  higher  we  ascend  the  rarer  it  becomes. 
It  may  therefore  be  considered  as  consisting  of  a  series  of  strata 
or  layers  K  G  I  M,  G  D  F  I,  D  A  C  F,  &c.,  (see  Fig.  26,) 
of  decreasing  density.  Now,  air,  as  well  as  all  transparent 
substances,  possesses  the  power  of  refracting  the  rays  of  light, 
or  bending  them  out  of  their  straight  course ;  thus  the  rays  of 
light  proceeding  from  a  star,  or  any  heavenly  body,  become 
bent  more  and  more  downwards  as  they  pass  through  the  at- 
mosphere, and  the  star  is  seen,  not  in  the  direction  in  which  it 
actually  lies,  but  in  the  direction  which  these  rays  have  at  the 
instant  of  arriving  at  the  eye  of  an  observer :  the  effect  of  this 
is,  to  cause  the  star  to  appear  higher  in  the  heavens  than  it 
really  is. 

In  Fig.  26,  let  S  represent  a  star  beyond  the  limits  of  the  atmosphere 
K  A  C  M  ;  S  B  m  the  straight  course  of  a  ray  of  light  proceeding  from 


Fig.  26.     Refraction. 

the  star.  In  passing  through  the  layer  of  atmosphere  A  C  F  D,  the  ray 
S  B  is  bent  down  into  the  direction  B  E  ;  now,  if  the  next  layer  D  F I  G 
were  of  the  same  density  as  A  C  F  D,  the  ray  B  E  would  proceed  in  the 
straight  line  B  E  n ;  but  as  the  former  is  denser  than  the  latter,  the  ray 
is  bent  down  into  the  direction  E  H ;  and  so  on  through  every  successive 


ASTRONOMY.  395 

layer,  until  the  ray  comes  to  the  eye  of  the  observer.  As  the  ray  of  light 
proceeds  downwards,  the  strata  of  air  become  more  and  more  dense,  which 
causes  the  ray  to  become  more  and  more  bent  in  its  passage  ;  hence  it  is 
that  the  course  of  the  refracted  ray  through  the  atmosphere  is  that  of  a 
curve,  which  becomes  more  and  more  concave  as  it  approaches  the  earth, 
as  shown  in  Fig.  27  ;  where  M  is  the  luminous  object ;  M  a  the  straight 


Fig.  27.    Atmospheric  Refraction. 

direction  of  the  rays  of  light,  which,  meeting  the  atmosphere  at  a,  are  by 
refraction  bent  into  the  curve  a  A  ;  A  m  is  the  direction  which  the  re- 
fracted ray  has  when  it  arrives  at  the  eye  of  the  observer  at  A ;  and  A  m 
is  the  direction  in  which  the  star  will  be  seen  :  thus  the  refraction  of 
the  atmosphere  causes  us  to  see  the  heavenly  bodies  apparently  higher 
above  the  horizon  than-  they  are  in  reality.  The  body  M  may  actually 
be  beneath  the  horizon,  and  yet  be  visible  to  a  person  at  A. 

The  atmospheric  refraction  elevates  the  apparent  position 
of  the  heavenly  bodies  most  when  they  are  near  the  horizon  ; 
and  at  the  zenith  it  does  not  affect  their  position  at  all. 

OVAL    FORM    OF    THE    SUN    AND    MOON    NEAR   THfc    HORIZON. 

65.  This  remarkable  appearance  is  occasioned  by  atmos- 
pheric refraction.     The  upper  half  of  the  sun  or  moon's  disk, 
as  the  case  may  be,  being  less  raised  by  refraction  than  the 
lower  half,  causes  the  vertical  diameter  of  the  disk  to  be  les- 
sened, while   the    horizontal    diameter   remains  unchanged ; 
hence  the  disk  appears  of  an  oval  shape,, 

TWILIGHT. 

66.  Twilight  is  that  light  which  we  enjoy  for  about  an  hour 
and  a  half  before  the  sun  has  appeared  above  the  horizon,  and 


396          NATURAL   AND   EXPERIMENTAL    PHILOSOPHY. 

for  about  the  same  time  after  he  lias  set.  This  beautiful  law 
of  nature  is  caused  by  the  reflection  of  the  sun's  light  from  the 
higher  regions  of  the  atmosphere.  Some  time  before  we  have 
any  direct  transmission  of  light  from  the  sun,  his  beams  illu- 
minate the  higher  portions  of  the  atmosphere,  and  then  this 
illuminated  portion  transmits  light  to  us. 

In  Fig.  28,  let  G  A  E  represent  the  earth  ;  G  K  C  D  E  a  portion  of 
its  atmosphere ;  A  the  place  of  an  observer  ;  A  II  his  horizon  ;  and  S 
the  sun  considerably  below  the  horizon,  and  of  course  invisible  to  a  person 


Fig.  28.     Cause  of  Twilight. 

at  A.  Now,  that  portion  of  the  atmosphere  represented  by  C  B  E  D  will 
be  illuminated  by  the  sun,  while  A  C  K  G  will  be  in  comparative  dark- 
ness ;  and  the  illuminated  portion  C  B  D  will  be  visible  to  a  person  at  A, 
and  the  light  proceeding  from  it  will  occasion  his  twilight.  The  duration 
of  twilight  varies  with  the  latitude  and  the  season  of  the  year.  At  the 
equator  the  duration  of  twilight  is  always  short,  whereas  at  the  poles  it 
lasts  for  upwards  of  four  months.  Twilight  begins  and  ends  when  the 
sun  is  about  eighteen  degrees  below  the  horizon. 


THE  TIDES. 

67.  The  alternate  flowing  and  ebbing  of  the  sea  is  called 
the  tides.  They  are  produced  by  the  attraction  of  the  sun 
and  the  moon  upon  the  waters  of  the  ocean,  but  chiefly  by 
the  attraction  of  the  moon  ;  for  as  she  is  much  nearer  to  the 
earth  than  the  sun,  her  attractive  force  upon  the  waters  is 
considerably  greater  than  that  of  the  sun's. 

For  a  little  more  than  six  hours,  the  sea,  in  certain  places, 


ASTRONOMY.  397 

gradually  swells  and  then  flows  into  harbors  and  the  mouths 
of  rivers  ;  this  is  called  flood  tide.  At  the  end  of  this  time 
the  ocean  has  attained  its  greatest  height ;  this  is  called  high 
water.  The  waters  then  begin  to  ebb  or  fall,  which  they  con- 
tinue to  do  for  a  little  more  than  six  hours,  until  they  arrive 
at  their  lowest  level ;  this  is  called  low  water.  Thus  the  wa- 
ters of  the  ocean,  day  after  day,  alternately  swell  and  fall  in  a 
little  more  than  six  hours ;  so  that  high  water  takes  place 
twice  in  every  24  hours  50  minutes,  this  being  the  time  which 
the  moon  takes  in  passing  from  the  meridian  of  a  place,  to 
returning  to  the  same  meridian  again.  If  the  moon  were 
stationary,  the  interval  between  high  water  of  one  day  and 
high  water  the  next  would  be  exactly  24  hours ;  for  the  same 
part  of  the  earth  would  return  to  the  moon's  meridian  in  this 
time  ;  but  while  the  earth  is  performing  a  revolution  on  its 
axis,  the  moon  advances  about  13°  in  her  orbit,  so  that  it  takes 
the  earth  about  50  minutes  more  to  bring  the  same  place  op- 
posite to,  or  on  the  same  meridian  with,  the  moon. 

68.  In  explaining  the  cause  of  the  tides,  we  shall  first 
speak  of  the  moon's  attraction  alone.  If  the  earth  were  an 
exact  sphere  covered  with  water,  and  if  there  were  no  exter- 
nal attraction  exerted  upon  it,  the  water  would  arrange  itself 
uniformly  over  the  surface,  forming  a  coating  like  the  rind  of 
an  orange  ;  but  when  the  earth  is  brought  under  the  influence 
of  an  attractive  body,  like  the  moon,  this  uniformity  in  the 
distribution  of  the  water  no  longer  subsists. 

In  Fig.  29,  let  E  represent  the  earth  surrounded  by  water ;  M  the 
moon ;  and  S  the  sun  ;  then,  since  the  moon's  attraction  is  greatest  upon 
the  objects  which  lie  nearest  to  her,  the  water  at  a,  directly  below  the 
moon,  will  be  more  attracted  by  her  than  the  -water  which  lies  farther 
off ;  hence  it  is  plain  that  the  water  at  a,  beneath  the  moon,  must  be 
drawn  up,  or,  as  it  were,  heaped  up ;  now,  as  the  earth  revolves  on  its 
axis,  successive  parts  of  its  surface  must  pass  under  the  moon,  and  these 
parts  will  have  high  water  in  regular  succession.  But  for  a  similar  rea- 
son there  will  also  be  high  water  at  c  on  the  opposite  side  of  the  earth ; 
for  the  water  at  c  must  be  less  drawn  towards  the  moon  than  the  water 
at  b  or  d,  or  any  parts  between  c  and  b,  or  c  and  d;  hence  it  follows  that 
34 


398          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  29.     Spring  Tide  at  New  Moon. 

the  water  must  be  heaped  up  towards  c.    At  b  and  d  there  will  be  low 
water. 

It  will  now  be  readily  seen  why  we  have  twice  high  water  and  twice 
low  water  in  the  course  of  every  24  hours  50  minutes.* 

69.  "We  have  hitherto  regarded  the  attraction  of  the  moon 
alone  as  the  cause  of  the  tides ;  but  this  is  not  strictly  true, 
for  the  sun's  attraction  very  much  affects  the  magnitude  of 
the  tides. 

The  largest  tides  take  place  when  the  moon  is  at  her  change, 
or  at  her  full  moon  ;  for  in  both  these  cases  the  attractive 
forces  of  the  sun  and  moon  combine  in  raising  the  waters ; 
these  are  called  spring  tides.  On  the  contrary,  the  lowest 
tides  take  place,  when  the  moon  is  at  the  beginning  of  her 
second  and  fourth  quarters,  that  is  to  say,  when  she  is  half 
moon  ;  for  then  the  attractive  forces  of  the  sun  and  moon  act 
so  as  to  diminish  each  other's  effect ;  these  are  called  neap 
tides. 

Fig.  29  represents  the  spring  tide  at  new  moon.  Here  the  attractive 
forces  of  the  sun  and  moon  obviously  cooperate  in  raising  the  waters  of 
the  ocean  at  a  and  c. 


*  It  must  be  observed  that  the  tide  is  not  at  its  highest  when  directly  under 
the  moon,  but  about  two  hours  later ;  for  since  the  full  effect  of  the  moon's 
attraction  on  the  waters  is  not  instantaneous,  high  water  will  not  take  place 
until  the  moon  has  passed  the  meridian  :  in  the  same  way,  the  hottest  part 
of  the  day  does  not  take  place  till  some  time  after  noon ;  and  also  the  month 
of  July  is  always  hotter  than  the  month  of  June. 


ASTRONOMY. 


399 


Fig.  30  represents  the  spring  tide  at  full  moon  ;  where  S  represents  the 
sun,  M  the  moon  at  her  full,  and  E  the  earth.    Here  the  attractive 


Fig.  30.     Spring  Tide  at  Full  Moon. 

forces  of  both  the  sun  and  the  moon  tend  to  draw  the  waters  away  from 
b  and  d,  and  to  accumulate  them,  or  heap  them  up,  at  a  and  c. 

Fig.  3 1  represents  the  neap  tides  at  half  moon  ;  where  M  represents 
the  moon,  either  at  the  beginning  of  her  second  or  at  the  beginning  of 


Fig.  31.    Neap  Tides. 

her  fourth  quarter.  Here  the  attraction  of  the  sun  tends  to  diminish  the 
flow  of  the  waters  at  b  and  c?,  and  hence  the  tides  at  these  periods  are 
smaller  than  at  any  other. 

Thus  in  the  course  of  a  lunar  month  we  have  two  spring  tides  and 
two  neap  tides. 


THE  FIXED  STARS. 

NUMBER    OF    THE   FIXED    STARS. 

70.   The  number  of  the  fixed  stars  exceeds  all  computation. 
Viewed  through  a  powerful  telescope,  the  milky  way,  or  galaxy, 


400          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

appears  like  great  groups  of  constellations.  Dr.  Herschel 
counted  600  stars  within  the  view  of  his  telescope  at  one  time ; 
and  in  one  portion  of  the  milky  way,  he  computed  that  the 
number  of  stars  exceeded  a  quarter  of  a  million.  But  if  the 
power  of  our  telescopes  were  still  further  increased,  there 
would  be  no  limit  to  the  number  of  stars  which  we  niio-ht 
observe. 


DISTANCE    OF  THE    FIXED    STARS. 

71.  It  has  been  ascertained  that  the  nearest  of  the  fixed 
stars  are  at  such  enormous  distances  from  us,  that  it  would 
take  their  light,  travelling  at  the  rate  of  twelve  millions  of. 
miles  in  a  minute,  at  least  six  years  in  reaching  us.  The  truth 
of  this  may  be  illustrated  in  the  following  manner :  — 

Look  at  two  trees  at  no  great  distance  from  you,  and  observe  the  ap- 
parent distance  between  them ;  now  change  your  position,  by  walking 
either  to  the  left  or  to  the  right,  and  observe  that  the  apparent  distance 
between  the  trees  is  decidedly  changed ;  indeed,  you  may  come  to  a 
position  where  the  two  trees  would  appear  as  one,  or,  more  correctly 
speaking,  in  the  same  straight  line.  Here,  then,  we  conclude  that  when 
objects  are  near  to  us,  their  apparent  distances  from  one  another  are  very 
much  affected  by  our  change  of  position.  Again,  proceeding  in  the 
same  manner,  look  at  two  objects  more  remote  from  you ;  then  you  will 
find  that  your  change  of  position  scarcely  at  all  alters  the  apparent  dis- 
tances of  the  objects.  Here,  then,  we  conclude,  that  when  objects  are 
very  distant  from  us,  their  apparent  distances  from  one  another  are  very 
little  affected  by  our  change  of  position.  Now,  the  earth,  as  it  revolves 
round  the  sun,  undergoes  a  change  of  position  measured  by  the  diameter 
of  its  orbit,  or  192  millions  of  miles.  The  earth,  therefore,  is  192  mil- 
lions of  miles  nearer  to  certain  fixed  stars  at  one  time  than  another ;  yet, 
notwithstanding  this  enormous  change  of  position,  there  is  scarcely  any 
difference  observed  in  the  apparent  distances  of  the  stars  from  one  an- 
other. How  immensely  great,  then,  must  their  distances  be  from  us !  * 

*  Astronomical  instruments  have  been  made  with  such  nicety,  that  a  dif- 
ference may  be  detected  in  the  apparent  distances  of  two  objects,  when  their 
distance  from  us  is  100,000  times  the  distance  between  the  two  points  of 
observation.  But  as  only  a  very  minute  difference  can  be  detected  by  our 
best  instruments  in  the  apparent  distances  of  the  stars  when  viewed  from 


ASTRONOMY.  401 

THE    STARS    HAVE    MOTION. 

72.  The  stars  have  a  motion  through  space  :  thus,  for  ex- 
ample, a  small  star  in  the  constellation  of  the  Swan  has  been 
found  to  move  annually  over  five  seconds  of  the  arc  of  the 
heavens.     Now,  according  to  Arago,  the  distance  of  this  star 
from  us  is  not  less  than  400,000  times  the  distance  of  the 
earth  from  the  sun :  in  order,  therefore,  that  this  star  should 
move  over  five  seconds  annually,  it  must  actually  travel  many 
millions  of  miles  in  this  time.     Hence  it  is  only  in  a  relative 
sense  that  we  can  speak  of  the  stars  as  being  fixed ;  abso- 
lutely considered,  there  is  probably  nothing  fixed  in  the  uni- 
verse. 

MULTIPLE   STARS.  —  GRAVITATION   EXTENDS  TO  THE   STARS. 

73.  Certain  stars,  although  they  appear  single  to  the  naked 
eye,  are   found  to  be  double  or   treble  stars  when  viewed 
through  a  good  telescope.     Stars  of  this  kind  are  very  numer- 
ous; in  120,000  stars  examined  by  M.  Struve,  one  in  every 
forty  was  found  to  be  a  multiple  star,  that  is,  a  group  of  two, 
three,  or  even  four  stars ;  indeed,  it  seems  probable  that,  were 
our  telescopes  sufficiently  powerful,  we  should  find  all  the 
stars  which   appear   single   to   the  naked   eye  to   be  really 
groups  of  stars. 

74.  In  these  multiple  stars  one  is  always  observed  to  be 
much  more  brilliant  than  the  rest.     This  brilliant  star  in  each 
group  is  the  central  sun,  round  which  the  others  revolve,  in 
the  same  manner  as  the  planets  in  our  system  revolve  round 
the    sun.     These   multiple    stars,  therefore,  are   systems  of 

the  opposite  points  of  the  earth's  orbit,  it  follows  that  the  nearest  stars  must 
be  at  least  100,000  times  192  millions  of  miles  from  us.  Mr.  Henderson  dis- 
covered that  the  star  called  Centauri  is  altered  in  its  apparent  position  by 
only  about  one  second ;  assuming  this  to  be  the  case,  the  distance  of  this 
star  from  us  must  be  about  half  a  million  of  times  the  earth's  distance  from 
the  sun.  This  angular  change  in  position  is  called  the  parallax  of  the  star. 
Not  more  than  ten  stars  have  at  present  been  found  to  have  any  parallax ; 
and  that  of  the  star  Centauri  is  the  greatest  which  has  yet  been  observed 
34* 


402  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

worlds  similar  to  our  solar  system,  thereby  proving  that  the 
law  of  gravitation,  which  animates  and  controls  the  planetary 
bodies,  exists  throughout  the  remote  regions  of  the  celestial 
spaces.  How  beautiful  it  is  thus  to  mark  the  unity  of  plan 
manifested  in  the  constitution  of  the  universe :  the  law  of 
attraction  which  causes  a  stone  to  fall  to  the  ground,  which 
gives  the  globular  form  to  the  mass  of  the  earth,  and  which 
guides  the  planets  in  their  motion  round  the  sun,  —  that  same 
law  binds  the  stars  to  one  another,  in  each  group  of  multiple 
stars ;  and  it  may  not  be  improbable  that  all  these  worlds  and 
systems  of  worlds  which  people  the  immensity  of  space  are 
but  parts  of  one  grand  integral  system,  which,  under  the  great 
controlling  principle  of  gravitation,  are  linked  to  one  another, 
as  well  as  to  one  vast  central  mass,  fixed  in  the  unfathomed 
depths  of  the  universe. 

"  That  very  law  which  moulds  a  tear, 

And  bids  it  trickle  from  its  source,  — 
That  law  preserves  the  earth  a  sphere, 
*   And  guides  the  planets  in  their  course." 


THE  DIVISIONS  OF  TIME.  — THE  CALENDAR. 

75.  The  motions  of  the  sun  and  moon  have  been  taken  in 
all  ages  as  the  measure  of  time.  The  diurnal  motion  of  the 
sun  is  the  measure  of  our  day ;  his  revolution  in  the  ecliptic 
gives  the  length  of  our  year  ;  and  the  periodic  return  of  new 
moon  is  the  basis  of  our  division  of  time  into  months. 


ASTRONOMICAL    AND    SIDEREAL    DAY. 

76.  The  astronomical  day  is  24  hours  long;  it  is  the  mean 
of  the  intervals  between  the  noon  of  one  day  and  the  noon  of 
the  succeeding  one. 

The  period  which  the  earth  takes  *to  revolve  on  its  axis  is 
constantly  the  same ;  viz.,  23  hours,  56  minutes,  4  seconds. 
This  is  called  a  sidereal  day,  for  it  is  the  time  which  any  me- 


ASTRONOMY.  403 

ridian  on  the  earth  takes  in  revolving  from  a  fixed  star  to 
that  star  again. 

The  astronomical  day  is  nearly  four  minutes  longer  than 
the  sidereal  day.  This  is  caused  by  the  sun's  motion  in  the 
ecliptic ;  for  while  the  earth  is  turning  on  its  axis,  the  sun  is 
advancing  amongst  the  stars,  and  hence  it  requires  the  earth 
to  make  rather  more  than  a  complete  revolution  to  bring  the 
same  meridian  under  him. 

EQUATION    OF   TIME. 

77.  Owing  to  certain  causes,*  which  need  not  at  present  be 
explained,  the  sun  does  not  move  uniformly  amongst  the  stars  ; 
and  hence  we  find  that  the  interval  between  two  successive 
noons   is   not  always  the  same.     A   clock,  therefore,  which 
keeps  true  time  will  not  always  correspond  with  the  time  as 
indicated  by  the  sun.     Thus,  for  example,  if  it  be  12  o'clock 
to-day  by  a  watch  keeping  true  time,  when  the  sun  is  exactly 
at  noon  or  on  the  meridian,  then  it  will  not  be  exactly  12 
o'clock  by  the  watch  to-morrow  when  the  sun  is  on  the  me- 
ridian ;  the  time  by  the  watch  may  be  a  little  before  or  after 
12  o'clock,  according  to  the  season  of  the  year.     This  differ- 
ence of  time  between  the  clock  and  the  sun  is  called  the  equa- 
tion of  time.     Almanacks  contain  the  amount  of  this  differ- 
ence for  every  day  of  the  year,  so  that  we  can  always  tell 
how  much  before  or  after  12  o'clock  the  sun  will  be  on  the 
meridian  on  any  proposed  day. 

SOLAR   YEAR.  —  JULIAN    CALENDAR. 

78.  As  the  return  of  the  sun  to  the  same  meridian  marks 
the  length  of  the  day,  so  the  return  of  the  sun  to  the  same 
equinox  gives  the  length  of  the  year. 

*  The  irregularity  of  the  sun's  apparent  motion  arises  from  the  following 
causes  :  First,  upon  the  inclination  of  the  ecliptic,  or  sun's  apparent  path,  to 
the  plane  of  the  equator ;  and  secondly,  upon  the  elliptic  form  of  the  earth's 
orbit,  which  occasions  the  earth  to  move  quicker  when  in  the  perihelion,  or 
nearest  the  sun,  and  slower  when  in  the  aphelion,  or  farthest  from  the  sun. 


404       •  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

The  solar  year  contains  365  days,  5  hours,  48  seconds,  or 
365  days,  6  hours,  nearly.  But  as  the  common  or  civil  year 
consists  of  only  365  days,  the  solar  year  is  about  a  quarter  of 
a  day  longer  than  the  civil  year ;  and  therefore,  if  this  year 
always  contained  365  days,  there  would  be  an  error  of  a  day 
committed  in  the  course  of  every  four  years.  Now,  in  order 
to  correct  this  error,  Julius  Caesar,  the  great  Roman  general, 
enacted  that  every  fourth  year  should  consist  of  366  days ; 
this  year  is  called  leap  year,  and  the  additional  day  is  added 
to  the  month  of  February,  which  therefore  consists  of  29  days 
in  leap  year.  This  mode  of  reckoning  is  called  the  Julian 
calendar. 

GREGORIAN    CALENDAR. 

79.  Now,  if  the  solar  year  had  consisted  of  365  days,  6 
hours,  exactly,  no  further  correction  would  have  been  neces- 
sary;  but  this  is  about  11  minutes  too  much,  and  consequently 
the  Julian  calendar  introduced  an  error  of  44  minutes  every 
4  years,  or  about  a  whole  day  in  130  years.  This  error  in 
the  course  of  centuries  became  considerable.  Thus,  in  the 
year  1577,  the  vernal  equinox  happened  on  the  llth  of 
March  in  the  place  of  the  21st.  Pope  Gregory,  in  the  year 
1582,  corrected  the  calendar  in  the  following  manner :  The 
5th  of  October  was  called  the  15th,  to  correct  the  error  which 
had  been  committed  since  the  time  of  Julius  Caesar ;  and  to 
prevent  the  error  happening  again,  it  was  agreed  that  every 
fourth  year  should  be  leap  year,  as  in  the  Julian  calendar, 
excepting  that  every  hundredth  year  for  three  successive  cen- 
turies, should  be  common  years,  and  the  fourth  hundredth 
should  be  a  leap  year.  Thus  1700, 1800,  and  1900,  are  com- 
mon years,  and  2000  is  a  leap  year.  By  this  mode  of  reck- 
oning, the  error  in  4000  years  will  not  exceed  one  day.  This 
is  called  the  Gregorian  calendar. 

The  Julian  calendar  is  called  the  old  style,  and  that  of  the 
Gregorian  the  new  style. 

The  Gregorian  calendar  was  at  once  received  by  all  Roman 


ASTRONOMY.  405 

Catholic  countries;  but  it  was  not  adopted  in  this  country 
until  the  year  1752.  The  Russians,  and  other  members  of 
the  Greek  church,  still  adhere  to  the  old  style,  or  the  Julian 
calendar. 

MODEL  EXERCISES. 

These  questions  are  not  only  intended  to  give  an  anal- 
ysis of  the  matter  going  before,  but  also,  by  a  suggestive 
course  of  reasoning,  to  lead  the  pupil  to  reflect  and  reason 
upon  the  knowledge  which  has  been  communicated  to  him,  and 
even  in  some  cases  to  extend  it. 


THE    STARS. 

Teacher.  What  is  the  point  directly  over  our  heads  called  ? 

Pupil.   The  zenith. 

T.  What  do  you  mean  by  the  horizon  ? 

P.  That  line  all  round  us  where  the  sky  and  the  earth  appear  to  meet. 

T.   What  shape  does  the  horizon  appear  to  have  ? 

P.   It  has  a  circular  shape,  and  bounds  our  view  on  all  sides. 

T.  What  point  in  the  heavens  is  that  which  lies  directly  below  our 
feet? 

P.   It  is  called  the  nadir. 

T.  What  would  our  zenith  be  to  a  person  living  on  the  opposite  side 
of  the  earth  ? 

P.   It  would  be  his  nadir. 

T.  If  I  cut  a  globe  (say  an  orange)  into  two  equal  parts,  what  is 
each  part  called  ? 

P.  Each  part  is  called  a  hemisphere,  or  half  sphere. 

T.   What  do  the  heavens  appear  like  ? 

P.   A  vast  dome,  or  concave  hemisphere. 

T.   Why  do  we  not  see  the  stars  during  the  day  ? 

P.   Because  of  the  superior  light  of  the  sun. 

The  teacher  should  continue  to  give  questions  of  this  kind,  taking 
care  to  vary  their  form,  until  the  pupil  is  thoroughly  master  of  the 
subject. 

CARDINAL    POINTS. 

Teacher.   In  what  part  of  the  heavens  does  the  sun  rise  ? 
Pupil.   He  rises  in  the  east,  and  sets  in  the  west. 


406          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

T.   At  noon  the  sun  shines  exactly  upon  the  front  of  my  house ;  now 
tell  me  the  direction  of  the  front  wall  of  my  house. 
P.   It  must  lie  in  a  line  extending  from  east  to  west. 
T.   "What  would  be  the  direction  of  each  gable  in  this  case  ? 
P.  Each  gable  wall  would  lie  in  a  line  extending  from  south  to  north. 


I 

B 

Fig.  32.     The  Cardinal  Points. 

T.  If  the  line  N  S  lies  north  and  south,  and  E  W  lies  east  and 
west,  how  do  these  lines  lie  with  respect  to  each  other  ? 

P.  They  lie  at  right  angles  to  each  other ;  that  is,  W  E  is  at  right 
angles  to  N  S  ;  or,  in  other  words,  W  E  is  perpendicular  to  N  S. 

T.   Describe  the  cardinal  points  in  a  map. 

P.  The  top  is  north,  the  bottom  south,  the  right  hand  east,  and  the 
left  hand  west. 

T.  What  will  be  the  direction  of  your  shadow  when  you  go  home 
to-night,  supposing  the  sun  to  be  shining  ? 

P.  It  will  be  cast  towards  the  east. 

And  so  on. 

DIURNAL   MOTION   OF  THE   HEAVENS.  —  MAGNITUDE  OP 
THE    STARS. 

Teacher.  What  is  meant  by  the  diurnal  motion  of  the  heavens  ? 

Pupil.   The  daily  revolution  of  the  heavens  about  the  polar  star. 

T.   In  what  direction  does  this  apparent  motion  take  place  ? 
«  P.  From  east  to  west.     (Why  ?)    Because  the  stars  appear  to  rise  in 
the  east  and  set  in  the  west. 

T.  What  do  you  mean  by  a  body  having  an  apparent  motion  ? 

P.  When  a  body  appears  to  us  as  if  it  moved,  without  really  doing 
so,  we  may  say  that  its  motion  is  only  apparent. 

T.  How  many  stars  may  be  seen  with  the  naked  eye  ? 

P.  About  two  thousand. 


ASTRONOMY.  407 

T.  What  stars  are  said  to  be  of  the  first  magnitude  ? 
P.  The  largest  and  brightest. 
T.   What  stars  belong  to  the  sixth  magnitude  ? 
P.   Those  which  are  just  visible  to  the  naked  eye. 
Proceed  with  the  remainder  in  the  same  manner. 

FIXED    STARS    AND    PLANETS.  —  CONSTELLATIONS. SIGNS 

OF    THE     ZODIAC. 

Teacher.  What  are  fixed  stars  ? 

Pupil.  Those  stars  which  do  not  change  their  distances  from  one 
another. 

T.  What  are  those  stars  called  which  do  not  always  remain  in  the 
same  place  ? 

P.  They  are  called  planets. 

T.  Which  stars  twinkle  most  ? 

P.  The  fixed  stars. 

T.  What  do  the  planets  look  like  when  viewed  through  a  good  tel- 
escope ? 

P.   They  look  like  little  luminous  balls. 

T.   What  is  a  constellation  ? 

P.   A  constellation  is  a  group  of  stars. 

T.  What  is  meant  by  the  two  pointers  in  Charles's  Wain  ? 

P.  Those  two  stars  in  the  back  of  the  supposed  wagon,  which  nearly 
point  towards  the  polar  star. 

T.   What  is  a  celestial  globe  ? 

P.  A  celestial  globe  represents  the  appearance  of  the  heavens,  with 
the  different  stars  and  constellations  marked  upon  it. 

T.  What  is  the  upper  star  in  the  pointers  called  ? 

P.  It  is  called  Dubhe. 

T.  Through  what  portion  of  the  heavens  do  the  planets  appear  to 
move? 

P.  Through  a  belt  or  band  of  stars,  containing  12  constellations, 
called  the  signs  of  the  zodiac. 

T.  What  is  the  ecliptic  in  the  heavens  ? 

P.  It  is  the  apparent  annual  path  of  the  sun.  It  is  marked  out  by 
the  constellations  of  the  zodiac. 

T.  Why  was  the  term  zodiac  given  to  these  constellations  ?  Can  you 
name  the  signs  of  the  zodiac  ? 


408     NATURAL  AND  EXPERIMENTAL  PHILOSOPHY. 


GENERAL  PRINCIPLES  OF  ASTRONOMY. 

Teacher.  Objects  appear  to  us  to  become  less  and  less  as  they  are  re- 
moved from  us.  Give  some  familiar  illustration  of  this.  Describe  the 
appearance  of  a  balloon  as  it  ascends. 

Pupil.  As  the  balloon  rises  in  the  air,  it  appears  to  us  to  become  smaller 
and  smaller,  until  at  length  it  gets  so  far  away  from  us  as  to  appear  very 
little  larger  than  a  foot  ball. 

T.  In  order,  therefore,  to  know  the  real  size  of  a  body,  we  must  not 
only  observe  its  apparent  size,  but  we  must  also  know  its  distance  from  us. 
Now,  let  there  be  two  trees  of  the  same  height,  and  suppose  one  of  them 
to  be  at  double  the  distance  of  the  other  ;  what  would  be  their  apparent 
magnitude  ? 

P.  The  more  distant  tree  would  appear  only  about  half  the  size  of  the 
other. 

T.  What  is  the  moon  ? 

P.  A  great  globe,  not  very  much  smaller  than  the  earth. 

T.  Why  does  she  appear  so  small  to  us  ? 

P.  Because  she  is  many  thousands  of  miles  from  us. 

T.  If  a  balloon  were  10  miles  from  us,  how  would  it  appear  ? 

P.  I  should  say  that  we  could  not  see  it  at  all ;  or,  in  other  words,  it 
would  be  invisible. 

T.  When  a  body  appears  to  move,  this  appearance  may  be  produced 
in  two  ways  ;  what  are  they  ? 

P.  First,  the  apparent  motion  may  be  produced  by  the  body  actually 
moving  in  the  direction  in  which  we  think  it  moves ;  and  secondly,  it 
may  be  produced  by  our  having  a  motion  in  a  direction  contrary  to  that 
in  which  the  body  appears  to  move. 

T.  What  have  you  to  say  relative  to  the  appearance  of  objects  when 
you  are  moving  in  a  railway  carriage?  The  heavens  appear  to  turn 
round  in  every  24  hours ;  how  may  this  be  explained  ?  What  is  the 
shape  of  the  earth  ?  In  what  time  does  it  turn  upon  its  axis  ?  What 
does  this  motion  of  the  earth  give  rise  to  ?  * 

T.  Give  me  a  familiar  instance  of  a  body  turning  or  spinning  round 
on  an  axis. 

P.  A  spinning  top. 

T.  Where  is  the  axis  in  this  case  ? 

P.  It  is  the  line  round  which  it  appears  to  spin. 

T.  The  earth  moves  round  the  sun  in  the  course  of  a  year  ;  how  does 
this  affect  the  appearance  of  the  sun  ? 

P.  It  gives  rise  to  the  apparent  motion  of  the  sun  in  the  ecliptic. 

*  The  pupil  is  supposed  to  answer  these  questions  in  succession. 


ASTRONOMY.  409 

T.  What  are  the  planets  ?  "What  do  they  revolve  round  ?  What  is 
the  sun  to  them  ?  Whence  do  they  derive  their  light  and  heat  ?  What 
is  the  path  of  a  planet  round  the  sun  called  ? 


SOLAR    SYSTEM. 

Teacher.  Give  a  familiar  example  of  one  body  revolving  round 
another. 

Pupil.  A  horse  revolving  round  a  man  in  the  centre  of  a  ring. 

T.  Of  what  does  the  solar  system  consist  ?  In  what  direction  do  the 
leading  planets  revolve  round  the  sun  ?  In  what  plane  do  the  orbits  of 
the  planets  nearly  lie  ?  In  what  direction  do  they  spin  round  on  their 
axes  ?  Name  the  planets  in  the  order  of  their  distances  from  the  sun. 
What  is  a  satellite  ?  How  many  primary  planets  are  there  at  present 
known  in  the  solar  system  ?  How  many  satellites  are  there  ?  Mention 
the  number  of  satellites  which  respectively  revolve  round  the  different 
planets,  &c. 

T.  If  I  move  this  orange  round  a  candle,  what  would  this  rudely  rep- 
resent ? 

P.  We  may  consider  the  candle  as  the  sun,  and  the  orange  as*  a  planet 
moving  round  him  in  its  orbit. 

T.  Now,  while  I  keep  the  orange  moving  round  the  candle,  suppose 
I  move  this  nut  round  the  orange  in  such  a  manner  that  the  nut  shall 
make  about  12  revolutions  round  the  orange  while  the  orange  makes  one 
revolution  round  the  candle  ;  what  would  this  rudely  represent. 

P.  It  would  represent  the  motion  of  the  earth  round  the  sun,  and  at 
the  same  time  the  motion  of  the  moon  round  the  earth. 

T.  What  are  comets  ?  Who  first  taught  correct  views  relative  to  the 
solar  system  ?  Who  was  Pythagoras  ?  Who  revived  the'  system  first 
taught  by  Pythagoras  ? 


THE    EARTH   AND    ITS   MOTION. FORM   AND    SIZE    OF 

THE    EARTH. 

Teacher.  Who  first  sailed  round  the  world  ? 

Pupil.   Magellan. 

T.  Who  first  made  the  attempt  ? 

P.  Columbus. 

T.  If  the  earth  were'  an  unbounded  flat  surface,  what  would  be  the 
consequence  of  a  vessel  constantly  sailing  from  any  place  ? 

P.  The  farther  the  vessel  sailed,  the  farther  she  would  get  away  from 
the  place. 

35 


410  NATURAL    AND    EXPERIMENTAL   PHILOSOPHY. 

T.  But  ships  never  sail  in  a  direct  line  from  any  place  ;  how  then  can 
they  be  said  to  sail  constantly  in  the  same  direction  ? 

P.  Ships  may  sometimes  go  to  the  right  or  to  the  left  of  their  direct 
course,  yet  still  they  pursue  a  certain  general  direction. 

T.  Just  in  the  same  way,  you  might  say,  that  a  little  fly  may  move 
round  this  globe,*  though  the  creature  may  go  in  a  zigzag  course. 
Why  do  we  not  see  the  hull  when  a  ship  has  sailed  some  distance 
from  us? 

P.  Because  the  round  part  of  the  earth's  surface  comes  between  us 
and  the  hull. 

T,  After  the  hull  of  a  ship  has  disappeared,  what  should  you  do  to 
get  a  sight  of  it  again  ? 

P;  I  should  get  to  the  top  of  some  high  tower  or  hill. 

T.  What  is  the  shape  of  the  earth  ? 

•P.  It  is  the  shape  of  a  ball  or  globe. 

T.  Some  boy,  I  think,  just  said  that  the  earth  is  round.  Now,  the 
upper  part  of  my  hat  is  round;  is  the  earth,  then,  the  shape  of 
my  hat  ? 

P.  Surely  not ;  the  earth  is  round  in  every  direction,  but  your  hat  is 
round  only  in  one  direction. 

T.  What  shape  does  my  hat  now  appear  to  have  ? 

P.  A  sort  of  oblong  shape. 

T.  How  do  you  know  that  the  earth  is  round  in  every  direction  ? 

P.  Because,  wherever  we  may  be,  we  always  find  that  the  horizon  has 
a  round  shape  ;  which  shows  that  the  earth  must  be  every  where  round 
to  present  this  appearance. 

T.  What  do  you  think  that  seamen  do  when  they  want  to  observe  a 
distant  sail? 

P.  They  climb  to  the  topmast. 

T.  Why? 

P.  That  they  may  see  a  greater  way  over  the  ocean. 

T.  (Moving  his  finger  round  the  globed  What  has  my  finger  moved 
over? 

P.  The  circumference  of  that  globe. 

T.  What  is  a  line  going  through  the  globe  called  ? 

P.  The  diameter. 

T.  If  the  line  only  went  to  the  centre,  what  would  it  then  be  called  ? 

P.  The  radius. 

T.  What  part  of  the  diameter  is  the  radius  ? 

*  In  giving  these  lessons,  the  teacher  must  be  provided  with  a  small  white 
globe,  having  a  rod  passing  through  it  to  represent  the  axis  of  the  earth,  and 
having  also  all  the  essential  lines  upon  the| errestrial  globe,  painted  in  strong 
black  lines. 


*        ASTRONOMY.'  411 

P.  One  half. 

T.  Now,  in  this  globe,  every  point  on  the  surface  is  at  the  same  dis- 
tance from  the  centre.  "What  have  you,  then,  to  say  respecting  the  radii 
of  a  globe  ? 

P.  That  they  are  all  equal  to  each  other. 

T.  How  many  times  is  the  circumference  of  a  globe  greater  than  the 
diameter  ? 

P.  A  little  more  than  three  times.* 

T.  If  the  length  of  a  line  stretching  from  London  to  York  be  200 
miles,  how  many  times  must  this  line  be  repeated  to  go  round  the 
earth? 

P.  About  125  times ;  because  25,000  divided  by  200  gives  125, 

T..  How  long  will  it  take  a  man  to  walk  round  the  earth,  supposing 
that  he  travels  25  miles  every  day  ? 

P.  About  1000  days,  or  nearly  three  years;  because  the  number  of 
miles  travelled  per  day  =  25  miles. 


DIURNAL  MOTION  OP  THE  EARTH. '—  LINES  UPON  THE 
GLOBE. 

.  Teacher.  How  much  of  the  earth's  surface  does  the  sun  enlighten  at 
one  time  ? 

Pupil.  One  half. 

T.  By  what  means  is  every  part  of  the  earth's  surface  brought  within 
the  light  and  heat  of  the  sun  ? 

P.  The  earth  is  made  to  turn  round  upon  its  axis  in  the  course  of 
every  day. 

T.  (Turning  a  globe  round.)  Now,  where  is  the  axis  in  this  revolving 
globe  ?  Is  there  a  real  axis,  or  only  an  imaginary  one  ? 

P.  The  axis  is -only  imaginary,  and  it  is  the  line  about,  which  the 
globe  appears  to  turn. 

T.  "What  have  you  now  to  say  respecting  the  axis  of  the  earth  ? 

P.  That  it  is  the  line  about  which  the  earth  appears  to  turn. 

T.  What  are  the  poles  upon  the  earth  ? 

P.  The  two  points  where  this  imaginary  axis  meets  the  earth's  surface. 

T.  On  what  point  is  my  finger  now  placed  ? 

P.  On  the  north  pole. 

T.  (Tracing  the  equator  with  his  pointer.)  What  is  this  line  called, 
and  how  is*  it  placed  with  respect  to  the  poles  ? 

P.  It  is  called  the  equator,  and  lies  at  the  same  distance  from  either 
of  the  poles. 

*  The  ratio  commonly  given  is  3*1416. 


412  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY.  ' 

T.  How  does  the  equator  divide  the  globe  ? 

P.  Into  two  equal  parts.  One  is  called  the  northern  hemisphere,  and 
the  other  the  southern  hemisphere. 

T.  Upon  what  hemisphere  is  my  hand  now  placed  ? 

P.  The  northern  hemisphere. 

T.  Is  there  any  other  way  in  which  the  changes  of  day  and  night 
might  be  produced  ? 

P.  Yes  ;  the  sun  might  turn  round  the  earth  in  the  course  of  a  day. 

T.  If  a  poor  woman  wanted  to  roast  a  joint  of  mutton  before  the  fire, 
what  would  she  do  in  order  to  have  every  part  equally  roasted  ? 

P.  She  would  tie  a  piece  of  string  to  t*he  mutton,  and  make  it  spin 
round  before  the  fire. 

T.  Is  there  any  other  way  in  which  this  might  be  done  ?   Now  think. 

P.  The  fire  might  be  made  to  turn  round  the  meat. 

T.  But  which  of  these  methods  is  the  better  ? 

P.  The  first  method,  certainly  ;  because  it  must  be  far  less  trouble  to 
make  the  meat  turn  round  before  the  fire  than  to  make  a  machine  for 
turning  the  fire  round  the  meat. 

T.  What  should  you  say  if  a  man  proposed  to  do  this  ? 

P.  That  although  he  might  show  some  ingenuity,  yet  he  would  be  a 
very  foolish  person. 

T.  Now,  it  is  equally  ridiculous  to  suppose  that  the  sun  turns  round 
the  earth.  It  is  too  monstrous  for  us  to  conceive  it  possible  that  Al- 
mighty God,  who  is  the  fountain  of  all  wisdom  and  goodness,  could  effect 
any  of  his  purposes  by  the  agency  of  means  which  it  would  appear  un- 
suitable, even  on  the  part  of  his  creatures,  to  employ. 


LATITUDE   AND   LONGITUDE. 

Teacher.  (Moving  his  pointer  round  the  globe.)  How  many  degrees 
have  I  moved  my  pointer  over  ? 

Pupil.  360°. 

T.  (Moving  his  pointer  from  the  pole  to  the  equator.)  How  many 
degrees  have  I  now  moved  my  pointer  over  ? 

P.  90°r  or  a  quadrant. 

T.  Why? 

P.  Because  it  is  a  quarter  of  the  whole  circumference,  and  the  quarter 
of  360°  will  be  90°. 

T.  Now,  knowing  the  circumference  of  the  earth  to  be  25,000  miles, 
I  want  you  to  tell  me  the  length  of  1°  ? 

P.  About  69£  miles  ;  because,  the  length  of  the  whole  circumference, 
or  360°,  being  25,000  miles,  the  length  of  1°  will  be  the  360th  part  of 
25,000  miles,  or  69£  miles  nearly. 


ASTRONOMY.  413 

T.  (Moving  his  pointer  over  a  meridian.)     What  is"  this  line  called  ? 

P.  It  is  a  meridian. 

T.  (Moving  his  pointer  on  the  equator,  between  the  first  meridian  and 
the  meridian  passing  through  a  place.)  What  is  this  distance  called  ? 

P.  The  longitude  of  the  place  through  which  the  meridian  passes.  t 

T.  (Putting  his  pointer  on  a  place  in  North  America.)  What  kind 
of  longitude  will  this  place  have  ? 

P.  West  longitude. 

T.  (Moving  his  pointer  on  a  parallel  of  latitude.)  What  is  this  line 
called? 

P.  A  parallel  of  latitude. 

T.  Why  is  it  called  a.  parallel  of  latitude  ? 

P.  Because  it  is  drawn  parallel  to,  and  even  with,  the  equator. 

T.  (Putting  his  pointer  on  a  place  in  the  southern  hemisphere.)  What 
kind  of  latitude  will  this  place  have  ? 

P.  South  latitude. 

T.  Here  is  a  meridian  passing  through  a  place.  Now,  if  this  distance 
(tracing  with  his  pointer  the  distance  between  the  place  and  the  equator) 
be  35°,  what  is  the  latitude  of  the  place  ?• 

P.  35°. 

T.  'Upon  what  line,  then,  is  the  latitude  of  a  place  measured  ? 

P.  Upon  a  meridian  line  passing  through  the  place. 

T.  How  many  things  must  be  given  to  fix  the  position  of  a  place  upon 
the  earth  ? 

P.  Two  things  :  the  longitude  and  latitude. 

T.  Is  this  parallel  of  latitude  a  great  or  small  circle  ? 


PROBLEMS    ON    LONGITUDE. 

Teacher.  When  it  is  noon  at  Greenwich,  what  time  will  it  be  to  a  place 
having  45°  west  longitude  ?  Ana.  Nine  o'clock  in  the  morning. 

T.  When  it  is  noon  at  Greenwich,  what  time  will  it  be  to  a  place 
having  60°  east  longitude?  Ans.  Four  o'clock  in  the  afternoon. 

T.  When  it  is  noon  with  us,  what  time  will  it  be  to  all  places  on  our 
opposite  meridian  ?  Ans.  It  will  be  midnight. 

T.  In  what  time  will  the  earth  turn  round  1°  ?    Ans.  4  minutes. 

Because,  tune  in  moving  round  360°  =  24  hours  ; 


T.  When  v  it  is  noon  at  Greenwich,  what  tune  will  it  be  to  a  place 
having  40°  east  longitude  ?  * 

It  has  been  shown  in  the  last  question  that  -places  having  a  difference 
35*  • 


414          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

of  1°  of  longitude  will  have  a  difference  of  four  minutes  in  time  ;  there- 
fore, a  difference  of  40°  in  longitude  will  have  a  difference  of  time  equal 
to  forty  times  four  minutes,  or  two  hours  and  forty  minutes.  But  as  the 
place  has  east  longitude,  it  will  have  its  noon  before  us,  and  consequently, 
when  it  is  noon  with  us,  it  will  be  two  hours  forty  minutes  past  noon  at 
the  place. 

T.  The  captain  of  a  ship  finds  that  the  pointer  of  his  clock,*  keeping 
Greenwich  time,  is  at  four  o'clock  in  the  afternoon  when  the  sun  is. in 
the  meridian  of  the  place  of  observation  ;  what  is  the  longitude  of  the 
ship  ?  Ans.  60°  west  longitude. 

T.  If  the  pointer  of  the  clock,  in  the  last  example,  be  at  seven  o'clock 
before  noon,  what  will  then  be  the  longitude  ?  Ans.  75°  east  longitude. 


THE    TROPICS    AND    ECLIPTIC. THE    ZONES. 

Teacher.  (Moving  his  pointer  on  the  tropic  of  Cancer.)  What  is  this 
line  called  ? 

Pupil.  The  tropic  of  Cancer. 

T.  When  does  the  sun  shine  perpendicularly  over  this  line  ? 

P.  On  our  midsummer  day,  or  the  21st  of  June. 

T.  (Moving  his  pointer  on  the  arctic  circle.)  What  is  this  line  called  ? 

P.  The  arctic  circle. 

T.  What  places  this  line  upon  the  globe  ? 

P.  The  fact  that,  on  our  midsummer  day,  the  sun's  light  extends  23^° 
over  the  north  pole. 

T.  (Moving  his  pointer  on  the  tropic  of  Capricorn.)  What  is  this 
line  called  ?  And  why  is  it  placed  here  ? 

P.  The  tropic  of  Capricorn.  The  sun  shines  perpendicularly  over  it 
on  our  midwinter  day,  or  the  21st  of  December. 

T.  (Tracing  out  the  torrid  zone.)     What  zone  is  this  ? 

P.  The  torrid  zone. 

T.  By  what  lines  is  it  bounded  ?  • 

P.  It  is  bounded  by  the  tropics  of  Cancer  and  Capricorn. 

T.  (Tracing  out  the  temperate  zone.)  What  zone  is  this,  and  how  is 
it  bounded  ? 

P.  It  is  the  temperate  zone,  and.it  is  bounded  by  the  tropic  of  Cancer 
and  the  arctic  circle. 

T.  How  many  zones  are  there,  and  what  are  they  called  ? 

P.  There  are  five  zones  : .  the  torrid,  the  two  temperate,  and  the  two 
frigid  zones. 


ASTRONOMY.  415 


ANNUAL  MOTION  OF  THE  EARTH. CAUSE  OP  THE 

SEASONS. 

Teacher.  (Moving  the  globe  round  the  candle,  &c.)  How  many  mo- 
tions has  this  globe  ? 

Pupil.  It  has  two  motions  :  one  on  its  axis,  and  the  other  round  the 
candle,  which  we  suppose  to  represent  the  sun. 

T.  What  are  these  two  motions  of  the  earth  called  ? 

P.  The  one  is  called  the  diurnal  motion,  and  the  other  the  annual 
motion. 

T.  Where  is  the  orbit  of  this  globe  ? 

P.  That  line  or  path  in  which  it  is  moving  round  the  candle. 

T.  (Bringing  the  globe  to  the  position  c.  See  Fig.  14,  p.  374.)  Now, 
when  the  earth  is  in  this  position,  what  season  have  we  ? 

P.  Summer,  because  the  sun  shines  more  over  the  northern  than  over 
the  southern  hemisphere. 

T.  (Holding  a  pointer  from  the  candle  to  the  tropic  of  Cancer  c.)  To 
what  point  on  the  earth's  surface  would  the  sun  be  now  shining  perpen- 
dicularly ? 

P.  To  a  point  in  the  tropic  of  Cancer. 

T.  How  much  on  every  side  of  this  point  will  the  sun's  light  extend  ? 

P.  It  will  extend  90°  over  the  earth  on  every  side,  because  the  sun 
enlightens  one  half  the  earth  at  one  time. 

T.  How  far  over  the  north  pole  will  his  light,  therefore,  at  this  time 
extend  ? 

P.  As  much  over  the  north  pole  as  the  tropic  of  Cancer  is  from  the 
equator,  that  is,  23£°. 

And  so  on  to  the  positions  d,  t,  and  6. 


Fig.  33.     Parallelism. 

Teacher.  (Moving  a  rod  without  changing  its  direction.)     What  have 
you  to  say  with  regard  to  the  position  of  this  rod  ? 


416          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Pupil.  That  although  it  is  being  moved  in  a  circle,  yet  it  still  main- 
tains its  parallelism. 

T.  Just  in  the  same  way,  you  might  say,  as  the  earth  preserves  the 
parallelism  of  its  axis  while  it  revolves  round  the  sun.  What  do  you 
mean  by  the  parallelism  of  the  earth's  axis  ? 

P.  That  it  is  always  parallel  to  itself,  or  that  it  constantly  lies  in  the 
same  direction. 

T.  (Moving  the  globe  round  the  candle,  with  the  axis  vertical.)  Why 
does  this  position  of  the  axis  not  account  for  the  seasons  ? 

P.  Because  the  sun  would  always  shine  perpendicularly  over  the 
equator,  and  therefore  both  hemispheres  would  always  enjoy  the  same 
amount  of  light  and  heat. 

T.  What  things  are  necessary  in  order  to  account  for  the  seasons  ? 


The  remainder  of  the  work  may  be  dissected  in  the  same  manner. 


ON  THE  USE  OF  THE  GLOBES. 

THE  TERRESTRIAL  GLOBE.    * 

DEFINITIONS    AND    EXPLANATIONS. 

1.  A  globe,  or  sphere,  is  a  round  body,  whose  surface  is 
every  where  at  the  same  distance  from  a  point  within  it  called 
the  centre. 

A  plane  passing  through  the  centre  of  a  sphere  divides  it 
'into  two  equal  parts,  called  hemispheres ;  and  the  section,  or 


Fig.  1.  A  Hemisphere.  Fig.  2.   A  Segment  of  a  Sphere. 

cut,  forms  a  great  circle  of  the  sphere.     All  great  circles  on 
the  same  sphere  are  obviously  equal  to  one  another. 

When  the  sphere  is  cut  by  a  plane  which  does  not  pass 
through  the  centre,  it  is  divided  into  two  unequal  parts,  and 
the  section  forms  a  small  circle  of  the  sphere.  The  size  of 
these  circles  depends  upon  the  distance  at  which  the  sphere  is 
cut  from  the  centre. 

If  a  circular  hoop  be  whirled  round,  it  will  describe  or  trace  out  the 
surface  of  a  sphere. 

(417) 


418 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Two  great  circles  on  a  sphere  di- 
vide each  other  into  equal  parts.  • 

These  circles  cross  each  other  like  two 
equal  hoops :  thus,  the  two  hoops  E  Q  and  E 
C  D  cross  each  other  at  the  points  A  and 
B,  making  A  C  B,  A  B  D,  A  E  33,  and 
A  B  Q  each  equal  to  semicircles.  The  pole 
of  a  great  circle  on  a  sphere  is  every  where 
90  degrees  distant  from  it. 


\ 


Fig.  3. 


2.  All  circles  on  the  globe  are  supposed  to  be  divided 
into  360  equal  parts,  as  in  Fig.  4, 

called  degrees.  Each  quadrant  of , 
the  circle  therefore  contains  90  de- 
grees. By  means  of  these  degrees 
the  magnitudes  of  angles  are  meas- 
ured :  thus,  for  example,-  the  angle 
A  C  K,  formed  by  the  two  lines 
A  C  and  C  K,  contains  40  degrees. 

3.  The  terrestrial  globe  is  made 
to  represent  the  earth.     Upon  the 


Fig.  4. 


•surface  of  this  globe  is  drawn  the  outline  of  the  land  and 
water,  according  to  their  relative  size  and  situation,  together 
with  the  various  lines  and  points  which  have  been  invented 
for  assigning  the  exact  position  of  a  place  upon  the  earth. 

4.  The  axis  of  the  earth  is  an  imaginary  line,  passing 
through  the  centre,  upon  which  the  earth  turns. 

This  line  is  represented,  in  the  artificial  globe,  by  the  wire  which 
passes  through  the  north  and  south  poles.  . 

5.  The  poles  of  the  earth  are  the  two  extremities  of  the 
axis.     One  pole  is  called  the  north  or  arctic  pole$  the  other, 
the  south  or  antarctic  pole. 

6.  The  equator  is  a  great  circle  passing  round  the  globe  at 
equal  distances  from  the  poles.     It  divides  the  globe  into  the 
northern  and  southern  hemispheres. ' 

The  equinoctial  is  the  equator  referred  or  extended  to  the 


ON  THE  USE  OF  THE  GLOBES.  419 

heavens.     When  the  sun  appears  in  the  equinoctial,  the  days 
and  nights  are  equal  all  over  the  world. 

7.  Meridians,  or  lines  of  longitude,  are  semicircles  extend- 
ing between  the  two  poles.     These  lines  cut  the  equator  at 
right  angles. 

The  meridian  passing  through  Greenwich  is  called  the  first 
meridian. 

8.  The  brazen  meridian  is  the  circle  of  brass  within  which 
the  artificial  globe  turns  on  two  axes  representing  the  poles 
of  the  earth.     One  half  of  the  brass  meridian  is .  graduated 
from  the  equator  to  the'  poles,  that  is,  the  point  over  the  equa- 
tor is  marked  0,  and  the  point  over  the  poles  is  marked  90  ; 
this  enables  us  to  find  the  latitude  of  a  place ;  the  other  half 
of  the  brass  meridian  commences  with  0  at  the  pole,  and  ends 
with  90  at  the  equator ;  this  enables  us  to  elevate  the  pole  to 
the  latitude  of  the  place. 

9.  The  longitude  of  a  place  is  the  distance  of  the  meridian 
passing  through  that  place  from  the  first  meridian,  reckoned 
in  degrees  on  the  equator.     Longitude  is  either  east  or  west, 
according  as  the  place  lies  to  the  east  or  west  of  the  first  me- 
ridian.    The  edge  of  the  brazen  meridian  is  usually  employed 
for  drawing  a  meridian  through  any  given  place. 

10.  Parallels  of  latitude  are  small  circles  drawn  parallel  to 
the  equator. 

The  polar  distance  of  a  place  is  its  distance  from  either  of 
the  poles. 

11.  The  latitude  of  a  place  is  its  distance  north  or  south 
from  the  equator,  reckoned  in  degrees  on  the  brass  meridian. 

12.  The  tropics  are  two  small  circles  drawn  parallel  to  the 
equator  at  the  distance  of  23£  degrees  from  it.     The  tropic 
in  the  northern  hemisphere  is  called  the  tropic  of  Cancer,  and 
that  in  the  southern  hemisphere  the  tropic  of  Capricorn. 

13.  The  polar  circles  are  two  small  circles  drawn  parallel 
to  the  equator  at  the  distance  of  23£  degrees  from  the  poles. 
The  north  polar  circle  is  called  the  arctic  circle,  and  the  south 
polar  one  the  antarctic  circle. 


420          NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

14.  The  zones.  —  The  earth  is  divided  by  the  tropics  and 
polar  circles  into  five  parts,  called  the  zones.     The  portion 
lying  between  the  tropics  of  Cancer  and  Capricorn  is  called 
the  torrid  zone  ;  between  the  tropic  of  Cancer  and  the  arctic 
circle,  the  north  temperate  zone  ;  between  the  tropic  of  Capri- 
corn and  the   antarctic  circle,  the  south  temperate  zone ;  be- 
tween the  arctic  circle  and  the  north  pole,  the  north  frigid 
zone;  between  the    antarctic  circle  and  the  south  pole,  the 
south  frigid  zone. 

15.  The  ecliptic  is  a  great  circle  representing  the  sun's  ap- 
parent path  throughout  the  year.     It  passes  through  the  trop- 
ics of  Cancer  and  Capricorn,  and  is  inclined  to. the  equator  at 
an  angle  of  23J-  degrees.     The  two  points  where  it  cuts  the 
equator,  or  equinoctial,  are  called  the  equinoctial  points. 

16.  Signs  of  the  zodiac.  —  The  ecliptic  is  divided  into  12 
equal  parts,  called  the  signs  of  the  zodiac ;  each  part  therefore 
'contains  30  degrees.     There  are  six  northern  signs  and  six 
southern  ones.     The  sun  appears  in  the  former  during  our 
spring  and  summer' months,  and  in  the  latter  during  our  au-. 
tumn  and  winter  months.     The  days  on  which  the  sun  enters 
the  different  signs  are  as  follows :  — 

Northern  Signs  of  the  Zodiac. 

Spring  Signs. 

cp  Aries,  the  Ram,  21st  of  March, 
y  Taurus,  the  Bull,  19th  of  April, 
rj  Gemini,  the  Twins,  20th  of  May. 

Summer  Signs. 

23     Cancer,  the  Crab,  21st  of  June. 

SI     Leo,  the  Lion,  22d  of  July. 

n$     Virgo,  the  Virgin,  22d  of  August. 

Southern  Signs  of  the  Zodiac. 
Autumnal  Signs. 

:£=  Libra*  the  Balance,  23d  of  September, 
m  Scorpio,  the  Scorpion,  23d  of  October. 
/  Sagittarius,  the  Archer,  22d  of  November. 


ON   THE    USE    OF    THE    GLOBES.  421 

"Winter  Signs. 

Vf     Capricornus,  the  Goat,  21st  of  December, 
•at     Aquarius,  the  "Waterman,  20th  of  January. 
H     Pisces,  the  Fishes,  19th  of  February. 

17.  The  equinoctial  points  (that  is,  the  two  points  where 
the  equator  cuts  the  ecliptic)  are  Aries  and  Libra.     The  for- 
mer point  is  called  the  vernal  equinox,  and  the  latter  the  au- 
tumnal equinox.     When  the  sun  is  in  either  of  these  points, 
the  days  and  nights  are  equal  all  over  the  world. 

18.  The  solstitial  points  are  Cancer  and  Capricorn.    When 
the  sun  is.  in  or  near  these  points,  the  variation  in  the  length 
of  the  days  is  scarcely  perceptible.     When  the  sun  enters 
Cancer,  it  is  the  longest  day  to  all  the  inhabitants  in  the  north- 
ern hemisphere,  and  the  shortest  day  to  those  in  the  southern 
hemisphere.     On  the  contrary,  when  the  sun  enters  Capri- 
corn, it  is  the  shortest  day  to  the  people  who  live  in  the  north- 
ern hemisphere,  and  the  longest  to  those  who  live  in  the  south- 
ern hemisphere. 

19.  The  colures  are  two  great  circles  which  pass  through 
the  poles ;  one  of  them,  called  the  equinoctial  colure,  passes 
through  the  equinoctial  points ;  the  other,  called  the  solstitial 
colure,  passes  through  the  solstitial  points. 

The  principal  lines  on  the  globe,  which  have  just  been  described,  are 
represented  in  the  annexed  figure ;  thus  N  S  represents  the  axis  of  the 
earth;  N,  the  north  pole;  S,  the  south  pole;  E  Q,*the  equator ;  E  Q  N, 
the  northern  hemisphere ;  E  Q  S,  the  southern  hemisphere ;  N  t  S,  a 
meridian ;  L  T,  a  parallel  of  latitude ;  L  N,  the  polar  distance  of  L ; 
c  v,  the  tropic  of  Cancer;  g  p,  the  tropic  of  Capricorn  ;  d  e,  the  arctic 
circle ;  f  g,  the  antarctic  circle ;  the  surface  of  the  earth  lying  between 
c  v  and  g  p,  the  torrid  zone ;  between  c  v  and  d  e,  the  north  temperate 
zone ;  between  d  e  and  the  north  pole,  the  north  frigid  zone ;  between  g  p 
and  f  q,  the  south  temperate  zone  ;  and  between  f  q  and  the  south  pole, 
the  south  frigid  zone ;  c  p,  the  ecliptic ;  C,  one  of  the  equinoctial  points; 
c  and  p,  the  solstitial  points ;  the  great  circle  N  C  S  going  round  the 
earth,  the  equinoctial  colure ;  and  N  c  S  v,  the  solstitial  colure. 

20.  The  zenith  is  that  point  in  the  heavens  directly  over 
our  heads. 

36 


422 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


South  Pole. 
Fig.  5. 

21.  The  nadir  is  that  point  in  the  heavens  which  lies  di- 
rectly below  our  feet. 

22.  Antipodes  are  those  people  who  live  on  opposite  sides 
of  the  earth,  and  therefore  walk  feet  to  feet.     Their  latitudes, 
longitudes,  days  and  nights,  seasons  of  the  year,  are  all  con- 
trary to  each  other. 

23.  The  horizon  is  of  two  kinds :  the  sensible  or  visible 
horizon  and  the  rational  or  true  horizon. 

The  sensible  or  visible  horizon  is  that  circle  on  the  earth 
which  bounds  our  riew. 

The  rational  or  true  horizon  is  a  great  circle  of  the  heav- 
ens, every  where  90  degrees  from  the  zenith.  The  stars  rise 
and  set  when  they  appear  on  this  line. 

24.  The  altitude  of  any  object  in  the  heavens  is  its  distance 
from  the  horizon.     When  the  body  is  on  the  meridian,  such 
as  the  sun  at  noon,  the  altitude  is  then  called  the  meridian 
altitude. 

25.  The  zenith  distance  of  a  celestial  body  is  its  distance 
from  the  zenith. 

26.  The  quadrant  of  altitude  is  a  thin,  flexible  slip  of  brass, 
divided  upwards  from  0  to  90  degrees,  and  downwards  from 


ON   THE    USE    OF    THE    GLOBES.  423 

0  to  18  degrees.  It  admits  of  being  screwed  to  the  brazen 
meridian.  The  upper  divisions  are  used  for  finding  the  dis- 
tances between  places  on  the  earth,  the  altitude  of  the  heav- 
enly bodies,  &c.,  and  the  lower  divisions  are  used  for  finding 
the  duration  of  twilight. 

27.  Azimuth  or  vertical  circles  are  great  circles  passing 
through  the  zenith  and  nadir  points,  cutting  the  horizon  at 
right  angles.     The  altitudes  of  the  heavenly  bodies  are  meas- 
ured on  these  circles.     This  is  done  by  screwing  the  quadrant 
of  altitude  on  the  zenith  of  the  place  of  observation,  and  mov- 
ing the  slip  of  brass  until  its  graduated  edge  passes  through 
the  body. 

28.  The  azimuth  of  any  celestial  body  is  an  arc  of  the  ho- 
rizon lying  between  a  vertical  circle  passing  through  the  body 
and  the  north  or  south  points  of  the  horizon. 

29.  The  amplitude  of  any  celestial  body  is  the  distance 
at  which  it  rises  from  the  east  or  sets  from  the  west. 

30.  The  cardinal  points  are  the  east,  west,  north,  and 
south  points  of  the  horizon. 

31.  A  mariner's  compass  consists  of  a  card,  representing  the 
horizon,  divided  into  thirty-two  equal  parts,  called  points  of 
the  compass,  together  with  a  magnetic  needle  which  always 
turns  its  north  pole  towards  the  north.     By  this  valuable  in- 
strument seamen  direct  the  course  of  their  ships,  and  engi- 
neers and  travellers  can  at  any  time  ascertain  the  cardinal 
points  of  the  horizon. 

The  needle  does  not  exactly  point  north  and  south.  In  England,  at 
the  present  time,  the  north  pole  of  the  needle  points  about  24  degrees  to 
the  westward  of  the  north.  Li  laying  down  a  meridian  line,  an  allow- 
ance must  be  made  for  this  variation. 

The  compass  is  placed  beneath  the  artificial  globe  for  setting  it  due 
north  and  south. 

32.  The  wooden  horizon,  surrounding  the  artificial  globe, 
represents  the  rational  horizon.     It  is  usually  divided  into 
seven  concentric  circles:  thej^rstf  is  for  finding  the  amplitude 
of  heavenly  bodies.     The  second,  for  finding  their  azimuth. 


424          NATURAL    AND   EXPERIMENTAL    PHILOSOPHY. 

The  third  contains  the  thirty-two  points  of  the  compass.  The 
fourth  contains  the  twelve  signs  of  the  zodiac,  with  the  de- 
grees of  each  sign.  The  fifth  contains  the  days  of  the  month, 
corresponding  to  every  degree  of  the  sun's  place  in  the  eclip- 
tic, as  indicated  in  the  fourth  circle.  The  sixth  contains  the 
equation  of  time,  that  is,  the  difference  of  time  between  a  clock 
and  a  sun  dial.  The  seventh  contains  the  twelve  calendar 
months. 

33.  The  hour  circle  is  a  flat  ring  of  brass,  turning  under 
the  brazen  meridian,  on  the  axis  or  pole  of  the  artificial  globe. 
It  is  divided  into  twenty-four  equal  parts,  representing  hours. 
It  is  used  for  finding  the  difference  of  time  between  any  given 
places,  the  length  of  the  day,  &c. 

34.  The  declination  of  the  sun  is  his  distance,  north  or 
south,  from  the  equinoctial.     At  the  equinoxes  he  has  no  dec- 
lination ;  at  the  tropic  of  Cancer  he  has  attained  his  greatest 
northern   declination  ;  and  at  the  tropic  of  Capricorn  he  has 
attained  his  greatest  southern  declination. 

35.  The  right  ascension  of  the  sun  is  the  distance  of  the 
meridian,  passing  through  the  sun's  place  in  the  ecliptic,  from 
the  equinoctial  point  Aries,  reckoned  in  degrees  eastward  on 
the  equator  or  equinoctial. 

36.  A  right  sphere  is  that  position  of  the  earth  where  the 
poles  are  in  the  horizon,  and  the  equator  passes  through  the 
zenith  and  nadir.     The  people  who  live  at  the  equator  have 
this  position  of  the  sphere. 

37.  A  parallel  sphere  is  that  position  of  the  earth  where 
the  poles  are  in  the  zenith  and  nadir,  and  the  equator  co- 
incides with  the  horizon.     If  there  were  any  people  living  at 
the  poles,  they  would  have  this  position  of  the  sphere. 

38.  An  oblique  sphere  is  that  position  of  the  earth  where 
the  equator  cuts  the  horizon  obliquely.     All  the  people  on  the 
earth  (excepting  those  that  live  at  the  equator  and  the  poles) 
have  this  position  of  the  sphere. 


ON   THE   USE    OF   THE    GLOBES.  425 


PROBLEMS  ON  THE  TERRESTRIAL  GLOBE. 

PROBLEM  I.  To  find  the  latitude  and  longitude  of  any 
given  place. 

RULE.  Bring  the  given  place  to  the  east  edge  of  the  brass 
meridian  :  the  degree  directly  over  the  place  is  the  latitude  ; 
and  the  degree  on  the  equator  cut  by  the  brass  meridian  is  the 
longitude. 

The  latitude  of  a  place  may  be  north  or  south,  and  the  lon- 
gitude east  or  west. 

EXAMPLES. 

1.  What  is  the  latitude  and  longitude  of  Paris  ? 
Ansicer.     48°  50'  north  latitude,  and  2°  20'  east  longitude. 
Required  the  latitudes  and  longitudes  of  the  following  places :  — 

2.  Rome ;   3.  South  Cape,  Spitzbergen ;   4.  Malta ;  5.  Cape  Horn ; 
6.  Azores. 

7.  What  is  the  latitude  and  longitude  of  the  north  pole  ? 

8.  What  is  the  greatest  latitude  a  place  can  have  ? 

9.  What  is  the  greatest  longitude  a  place  can  have  ? 

10.  What  part  of  the  earth  is  that  which  has  no  latitude  ? 

ANSWERS. 

(2.)  41°  54'  N.  lat,  and  12°  27'  E.  long. 
(3.)  76°  32'  N.  lat.,  and  13°  45'  E.  long. 
(4.)  35°  53'  N.  lat.,  and  14°  30'  E.  long. 
(5.)  55°  58'  S/lat.,  and  67°  11'  W.  long. 
(6.)  39°  N.  lat.,  and  28°  W.  long. 

(7.)  90°  N.  lat.;  (8.)  90°;  (9.)  180°  east  or  west  longitude;  (10.) 
The  equator. 

PROBLEM  II.  To  find  any  place  on  the  globe,  having  its 
latitude  and  longitude  given. 

RULE.  Find  the  given  longitude  on  the  equator,  and  bring 
it  to  the  brass  meridian  ;  find  the  given  latitude  on  the  brass 
meridian,  and  the  place  immediately  under  will  be  the  place 
required. 

EXAMPLES. 

(1.)  What  place  has  20°  north  latitude,  and  76°  west  longitude? 
Answer.    The  Island  of  Cuba. 
36* 


426          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

What  places  have  nearly  the  following  latitudes  and  longitudes  ? 
(2.)  54°  N.  lat.,  and  18£°  E.  long. 
(3.)  30°  N.  lat.,  and  31°  E  long. 
(4.)  21°  S.  lat.,  and  55£°  E.  long. 
(5.)  29°  N.  lat.,  and  18°  W.  long. 
(6.)  34°  S.  lat.,  and  18°  E.  long. 

Answers.  (2.)  Dantzic ;  (3.)  Cairo ;  (4.)  Island  of  Bourbon ;  (5.) 
Canary  Islands,  Palma  ;  (6.)  Cape  of  Good  Hope  town. 

PROBLEM  III.  To  find  all  those  places  which  have  the 
same  latitude  as  a  given  place. 

RULE.  Bring  the  given  place  to  the  brass  meridian,  and 
find  its  latitude ;  turn  the  globe  slowly  round,  and  all  places 
which  pass  under  the  observed  latitude  will  be  those  required. 

All  places  in  the  same  latitude  have  the  same  seasons,  and 
the  same  length  of  day  and  night ;  but,  owing  to  various  physi- 
cal causes,  (such  as  the  relative  distribution  of  land  and  water,) 
they  may  not  have  the  same  temperature. 

EXAMPLES. 

1.  What  places  have  nearly  the  same  latitude  as  Constantinople  ? 
Answer.     Naples,  Pekin,  Philadelphia,  &c. 

What  places  have  nearly  the  same  latitude  as  the  following  : 

2.  London ;  3.  Alexandria ;  4.  Rome  ? 

5.  What  places  have  nearly  the  same  length  of  days  as  Malta  ? 
Answers.     (2.)  Rotterdam,  &c. ;  (3.)  Cummin's  Island,  China,  &c. ; 
(4.)  Nova  Scotia  ;  (5.)  Cape  St.  Vincent,  Portugal,  &c. 

PROBLEM  IV.  To  find  all  those  places  which  have  the 
same  longitude  as  a  given  place. 

RULE.  Bring  the  given  place  to  the  brass  meridian ;  all 
places  under  the  edge  of  the  brass  meridian,  from  pole  to  pole, 
have  the  same  longitude. 

The  people  living  in  all  those  places  which  have  the  same 
longitude,  have  noon  and  all  other  hours  of  the  day  alike. 

EXAMPLES. 

1.   What  places  have  nearly  the  same  longitude  as  Madeira  ? 
Answer.    Hecla,  Teneriffe,  Cape  Blanco,  &c. 


ON    THE    USE    OF    THE    GLOBES.  427 

2.  What  inhabitants  of  the  earth  have  nearly  the  same  time  as  the 
people  of  the  Cape  of  Good  Hope  ? 

3.  What  places  have  nearly  the  same  longitude  as  Gibraltar  ? 
Answers.     (2.)    Dantzic,   Stockholm,  &c.  ;    (3.)  St.  David's  Head, 

Wales,  &c. 

PROBLEM  V.     To  find  the  distance  between  two  places. 

RULE.  Lay  the  edge  of  the  quadrant  of  altitude  over  the 
two  places,  so  that  the  point  marked  0  may  be  over  one  of 
them  ;  then  the  number  of  degrees  over  the  other  place  will 
give  the  number  of  degrees  that  they  are  apart. 

Multiply  the  number  of  degrees  by  60,  and  the  product  will 
give  the  geographical  miles  ;  or  multiply  the  number  of  de- 
grees by  69^,  and  the  product  will  give  the  distance  in 
English  miles. 

Or,  take  the  distance  between  the  two  places  with  a  thread, 
apply  .that  distance  to  the  equator,  and  it  will  show  how  many 
degrees  are  contained  in  the  distance. 

EXAMPLES. 

1.  What  is  the  distance  between  London  and  Madeira  ? 

Answer.  About  22£°,  or  1350  geographical  miles,  or  about  1554 
English  miles. 

What  is  the  distance  between  the  following  places  ? 

2.  London  and  Constantinople. 

3.  Cape  Verd  Isles  and  the  Cape  of  Good  Hflpe. 

4.  London  and  Petersburg. 

5.  What  is  the  distance  of  Land's  End  from  Jamaica  ? 

6.  Suppose  a  ship  to  sail  from  Liverpool  to  Madras  in  the  following 
track  :  from  Liverpool  to  Cape  Verd  Islands,  thence  to  St.  Helena, 
thence  to  the  Cape,  thence  to  Mauritius,  thence  to  Ceylon,  and  thence  to 
Madras ;  how  many  English  miles  are  there  in  the  voyage  ? 

ANSWERS. 

(2.)  1320  geog.  miles,  and  1535  Eng.  miles. 
(3.)  3900  geog.  miles,  and  4491  Eng.  miles. 
(4.)  1140  geog.  miles,  and  1312  Eng.  miles. 
(5.)  3840  geog.  miles,  and  4421  Eng.  miles. 
(6.)  About  185°,  or  11,100  geog.  miles,  or  about  12,783  Eng.  miles. 

PROBLEM  VI.     The  hour  of  the  day  being  given  at  one 
place,  to  find  what  hour  it  is  at  any  other  place. 


428  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

RULE.  Bring  the  place  at  which  the  time  is  given  to  the 
brass  meridian ;  set  the  hour  index  to  the  given  hour ;  turn 
the  globe  until  the  other  place  is  brought  under  the  brass  me- 
ridian, and  the  index  will  point  to  the  required  time. 

Or  thus  by  calculation.  Find  the  difference  of  longitude 
between  the  two  places,  allow  an  hour  for  every  15  degrees, 
and  four  minutes  of  time  for  every  degree,  and  the  time  thus 
obtained  will  give  the  difference  of  time  between  the  two 
places.  If  the  place  at  which  the  time  is  required  lies  to  the 
east  of  the  other  place,  this  difference  of  tune  must  be  added 
to  find  the  tune  at  the  place  required ;  but  if  to  the  west,  it 
must  be  subtracted.  See  ASTRONOMY,  Art.  22,  and  EXER- 
CISES, p.  413. 

EXAMPLES. 

1.  When  it  is  4  o'clock  in  the  afternoon  at  London,  what  time  is  it  at 
Petersburg  ? 

Answer.     Six  o'clock  in  the  evening. 

Or  thus,  more  accurately,  by  calculation.  The  difference  of  longitude 
between  London  and  Petersburg  is  30°  25'.  Here  the  30  degrees  exact- 
ly give  2  hours  difference  of  time,  and  to  convert  the  remaining  25'  into 
time,  we  have 

No.  min.  of  time  corresponding  to  25'  =  —  X  4  =-  =  If , 

which,  added  to  the  2  hours,  gives  2  hours  1|  min.  for  the  difference  of 
tjme. 

Now,  as  Petersburg  lies  to  the  east  of  London,  the  time,  at  the  former 
place  will  be  2  hours  If  min.  later  than  it  is  at  London ;  that  is,  the  time 
at  Petersburg  will  be  If  min.  past  six  in  the  evening. 

2.  When  it  is  1  o'clock  in  the  afternoon  at  Alexandria,  what  time  is 
it  at  Philadelphia  ? 

Answer.     Seven  o'clock  in  the  morning. 
Or  thus,  more  accurately,  by  calculation. 
Longitude  of  Alexandria  =  30°  16'  east. 
Longitude  of  Philadelphia  =  75°  19'  west. 

Difference  of  longitude    =  105°  35' 

105 

Difference  time  in  hours  =  — —  =  7  hours. 
15 

35  7 

Difference  time  in  min.      =5-X4  =  -=2J  min. 

oU  o 

Total  difference  of  time  =  7  hours  2  J  minutes. 


ON  THE  USE  OF  THE  GLOBES.  429 

Now,  as  Philadelphia  lies  to  the  west  of  Alexandria,  the  time  of  the 
former  place  will  be  7  hours  2£  min.  earlier  than  it  is  at  the  latter  place ; 
hence  the  time  at  Philadelphia  will  be  57f  min.  past  5  in  the  morning. 

3.  When  it  is  4  o'clock  in  the  afternoon  at  Cape  Horn,  what  time  is 
it  at  the  Island  of  St.  Helena  ? 

4.  When  it  is  10  o'clock  in  the  morning  at  Nankin,  in  China,  what 
time  is  it  at  Plymouth,  England  ? 

ANSWERS. 

(3.)  6  min.  past  8  o'clock  in  the  evening  nearly. 
(4.)  |  past  1  o'clock  in  the  morning  nearly. 

PROBLEM  VII.  Given  the  difference  of  time  at  any  two 
places  to  find  their  difference  of  longitude. 

RULE.  Bring  the  first  meridian  to  the  brass  meridian ;  set 
the  hour  index  at  12  o'clock;  turn  the  globe  until  the  given 
time  is  brought  under  the  brass  meridian  ;  and  the  degree  of 
the  equator  cut  by  the  brass  meridian  will  be  the  difference 
of  longitude. 

Or  thus  by  calculation.  Allow  15  degrees  difference  of  lon- 
gitude for  every  hour  in  the  difference  of  time,  or  1  degree 
for  every  4  minutes  of  time. 

EXAMPLES. 

1.  When  it  is  noon  at  a  certain  place,  it  is  8  o'clock  in  the  morning  at 
London  ;  required  the  longitude  of  the  place. 

Answer.     60°  east  longitude. 

Or  thus  by  calculation.    Here  the  difference  of  time  is  4  hours. 
Difference  longitude  =  4  X  15  =  60  degrees. 

As  the  time  at  London  is  before  that  of  the  place,  it  follows  that  it 
must  have  60  degrees  east  longitude. 

2.  When  it  is  10  o'clock  in  the  morning  at  London,  at  what  places 
will  it  be  noon  ? 

3.  What  places  will  have  noon  7  hours  55  min.  before  London  ? 

ANSWERS. 

(2.)  To  all  places  having  30°  E.  long.,  —  Petersburg,  &c. 
(3.)  To  all  places  having  118$°  E.  long.,  —  Nankin,  &Q. 

PROBLEM  VIII.  To  jind  the  length  of  a  degree  in  any 
given  parallel  of  latitude. 


430  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

RULE.  Lay  the  edge  of  the  quadrant  of  altitude  parallel 
to  the  equator  between  any  two  meridians,  (15  degrees  of 
longitude  apart ;)  then  the  number  of  degrees  intercepted  be- 
tween them,  multiplied  by  4,  will  give  the  number  of  geograph- 
ical miles  contained  in  a  degree  of  the  given  parallel  very 
nearly.  To  find  the  number  of  English  miles,  multiply  the 
geographical  miles  by  69.1  and  divide  by  60. 

EXAMPLES. 

1.  How  many  geographical  and  English  miles  are  there  contained  in 
a  degree  in  the  latitude  of  40°  ? 

Here  the  distance  between  two  meridians  (15  degrees  apart)  in  the 
parallel  of  40°,  is  ll£  degrees  of  the  equator  nearly  ;  hence  we  have 
Length  of  15  degrees  longitude  on  parallel  40° 
=  ll£  degrees  of  the  equator 
=  11£  X  60  geographical  miles  ; 
Length  of  one  degree  longitude  on  parallel  40° 

Hi  X  60 

llj|  x  4  =  46  geog.  miles 


15 

46  X  69.1 
60~~ 


Eng.  miles  =  52.97  Eng.  miles. 


How  many  geographical  and  English  miles  are  there  contained  in  a 
degree  in  the  following  latitudes  ? 

(2.)  30°;  (3.)  51°;  (4.)  56°;  (5.)  60°. 

ANSWERS. 

(2.)  51.9  geog.  miles,  or  59.7  Eng.  miles. 
(3.)  37.7  geog.  miles,  or  43.4  Eng.  miles. 
(4.)  33.5  geog.  miles,  or  38.5  Eng.  miles. 
(5.)  30  geog.  miles,  or  34£  Eng.  miles. 

PROBLEM  IX.     To  find  the  antipodes  of  a  given  place. 

RULE.  Place  the  two  poles  of  the  globe  in  the  horizon ; 
turn  the  globe  until  the  given  place  comes  to  the  eastern  part 
of  the  horizon  ;  observe  the  number  of  degrees  that  the  place 
is  to  the  north  (or  south)  of  the  east  point  of  the  horizon,  and 
the  same  number  of  degrees  counted  south  (or  north)  from 
the  west  point  of  the  horizon  will  give  the  antipodes  required. 


ON   THE   USE    OF   THE   GLOBES.  431 

EXAMPLES. 

1.  Required  the  antipodes  of  London. 

Answer.    Antipodes  Island,  near  the  Island  of  New  Zealand. 

Required  the  antipodes  of  the  following  places :  2.  The  Island  of 
Bermudas;  3.  Cape  Horn;  4.  Cape  of  Good  Hope;  5.  the  Azores. 

Answers.  (2.)  The  south-west  part  of  New  Holland ;  (3.)  the  east 
of  Lake  Baikal;  (4.)  the  north  of  the  Sandwich  Islands;  (5.)  east 
of  Cape  Howe. 

PROBLEM  X.     To  rectify  the  globe  for  a  given  place. 

RULE.  Elevate  or  raise  the  corresponding  pole  as  many 
degrees  above  the  wooden  horizon  as  are  equal  to  the  latitude 
of  the  .place.  See  ASTRONOMY,  Art.  28. 

If  the  globe  be  now  turned  round,  so  as  to  bring  the  place 
to  the  brass  meridian,  it  will  be  seen  that  the  place  occupies 
the  zenith  of  the  globe ;  that  is  to  say,  the  wooden  horizon 
forms  the  true  horizon  to  the  place. 

PROBLEM  XL  To  find  the  sun's  place  in  the  ecliptic  for 
any  given  day. 

RULE.  Find  the  month  and  the  mark  corresponding  to  the 
day  of  that  month  in  the  outer  circle  of  the  wooden  horizon ; 
then  the  coincident  mark  in  the  circle  containing  the  signs  of 
the  zodiac  will  give  the  sun's  place  in  the.  ecliptic,  which  may 
then  be  found  upon  the  globe. 

PROBLEM  XII.  To  find  the  sun's  declination  for  a  given 
day  of  a  given  month,  and  to  find  the  places  to  which  the  sun 
will  be  vertical  on  that  day. 

RULE.  Find  the  sun's  place  in  the  ecliptic  for  the  given 
day,  (Prob.  XI. ;)  bring  that  point  of  the  ecliptic  to  the  brass 
meridian,  and  the  degree  directly  over  it  on  the  brass  merid- 
ian is  the  declination  north  or  south.  Turn  the  globe  round, 
and  every  place  which,  passes  under  thafc  degree  of  the  brass 
meridian  will  have  the  sun  vertical  on  that  day. 

The  declination  of  the  sun  obviously  gives  the  latitude  of 
the  places  which  will  have  the  sun  vertical. 

The  sun  can  only  be  vertical  to  places  lying  within  the 
torrid  zone. 


432          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

EXAMPLES. 

1.  Required  the  declination  of  the  sun  on  the  llth  of  July.     What 
will  be  the  latitude  of  the  places  to  which  the  sun  will  be  vertical  on 
that  day  ? 

Answer.  22°  north  declination ;  and  22°  north  latitude,  Lessoe  Isl- 
and, Cattegat,  &c. 

To  find  the  declination  and  the  places  to  which  he  will  be  vertical  on 
the  following  days :  — 

2.  3d  of  October;   3.  24th  of  July;   4.  10th  of  January;   5.  10th  of 
June. 

Ansicers.     (2.)   4°  south  decl.,  and  4°  south  latitude.  . 
(3.)   20°  north  decl.,  and  20°  north  latitude. 
(4.)   22°  south  decl.,  and  22°  south  latitude. 
(5.)  23°  north  decl.,  and  23°  north  latitude. 

PROBLEM  XIII.  To  find  the  hour  at  which  the  sun  rises 
and  sets  at  a  given  place,  for  any  given  day. 

RULE.  Rectify  the  globe  for  the  latitude  of  the  place, 
(see  Prob.  X. ;)  find  the  sun's  place  in  the  ecliptic,  (see 
Prob.  XI.,)  and  bring  it  to  the  brass  meridian.  Set  the  in- 
dex of  the  hour  circle  to  XII. ;  turn  the  globe  till  the  sun's 
place  comes  to  the  eastern  edge  of  the  wooden  horizon,  and 
the  index  will  show  the  hour  at  which  the  sun  rises ;  then 
turn  the  globe  till  the  sun's  place  comes  to  the  western  edge 
of  the  wooden  horizon,  and  the  index  will  show  the  hour  at 
which  the  sun  sets. 

The  length  of  the  day  is  found  by  doubling  the  hour  of 
sunset. 

The  amplitude  of  the  sun  will  be  found  by  simply  ob- 
serving the  point  on  the  wooden  horizon  which  is  cut  by  the 
sun's  place  in  the  ecliptic  at  the  time  of  rising  or  setting. 

EXAMPLES. 

1.  At  what  time  will  4he  sun  rise  and  set  to  the  people  of  London  on 
the  21st  day  of  December?    Required  also  the  sun's  amplitude  on  this 
day. 

Answer.  Rises  \  before  8,  and  sets  £  past  4  ;  the  amplitude  about  36° 
to  the  south  of  the  east  point  of  the  horizon. 

2.  At  what  time  will  the  sun  rise  and  set  to  the  people  of  Rome  on 
the  1st  day  of  April,  &c.  ? 


ON    THE    USE    OF    THE    GLOBES. 

3.  What  is  the  length  of  the  longest  day  to  the  inhabitants  of  Paris  ? 
At  what  distance  from  the  east  point  of  the  horizon  does  the  sun  rise  on 
this  day  ? 

.  4.  Show  that  the  day  is  always  12  hours  long  to  the  people  living  at 
the  equator.  Show  that  the  21st  of  June  is  the  longest  day  to  the-  in- 
habitants of  the  northern  hemisphere,  and  that  the  21st  of  December  is 
their  shortest  day. 

Required  the  length  of  the  shortest  day  to  the  inhabitants  of  the  fol- 
lowing places  :  5.  Edinburgh ;  6.  New  York. 

Answers.  (2.)  Rises  3  before  6,  and  sets  |  after  6 ;  amplitude  5° 
north  of  the  east  point;  (3.)  Length  of  the  day  16  hours,  and  about 
37°  north  of  the  east  point;  (5.)  6£  hours;  (6.)  9  hours. 

PROBLEM  XIV.  To  find  the  sun's  meridian  altitude  at  a 
given  place  on  a  given  day. 

RULE.  Rectify  the  globe  for  the  latitude  of  the  place; 
bring  the  sun's  place  in  the  ecliptic  for  the  given  day  to  the 
brass  meridian ;  count  the  number  of  degrees,  on  the  brass 
meridian,  between  that  place  and  the  horizon  for  the  meridian 
altitude  required. 

Or  thus.  Find  the  declination  of  the  sun,  and  add  it  to  the 
co-latitude  of  the  place  when  the  declination  and  latitude  are 
of  the  same  name,  but  subtract  it  when  they  are  of  different 
names. 

EXAMPLES. 

1.  What  is  the  sun's  meridian  altitude  at  London  on  our  midsummer 
day? 

Answer.  62°.  This  is  the  greatest  elevation  of  the  sun  above  the 
horizon  of  London. 

Or  thus  by  calculation.  Here  the  declination  and  latitude  are  of  the 
same  name.  On  this  day  the  declination  of  the  sun  is  23£°  north,  and 
the  co-latitude  is  90°  less  by  5l£°,  or  38£°  ;  hence  the  meridian  altitude 
=  38£°  -f  23i  =  62°. 

2.  What  is  the  sun's  meridian  altitude  at  London  on  our  midwinter 
day? 

Answer.  15°.  This  is  the  least  meridian  altitude  of  the  sun  to  the 
inhabitants  of  London. 

Or  thus  by  calculation.  Here  the  declination  and  latitude  have  dif- 
ferent names.  In  this  case,  therefore,  we  have  the  meridian  altitude  = 
3SA  —  2:U  =  15°. 

37 


434          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

3.  Required  the  sun's  meridian  altitude  at  Paris  on  the  1st  of  August. 

4.  What  is  the  sun's  meridian  altitude  at  London  on  the  2d  of  Feb- 
ruary ?         , 

What  would  be  the  meridian  altitude  of  the  sun  on  the  21st  of  June 
to  the  following  places?  5,  The  north  pole;  6.  The  arctic  circle;  7.  The 
equator. 

Answers.  (3.)  59°  10'  ;  (4.)  2l£°;  (5.)  23i° ;  (6.)  47°;'  (7.)  66£0 
or  23£°  from  the  zenith. 

PROBLEM  XV.  To  find  the  altitude  of  the  sun  at  any 
given  place  and  hour  ;  and  also  his  azimuth. 

RULE.  Rectify  the  globe  for  the  latitude  of  the  given 
place ;  bring  the  sun's  place  to  the  brass  nieridhin ;  set  the 
index  to  XII. ;  turn  the  globe  till  the  index  points  at  the 
given  hour ;  fix  the  quadrant  of  altitude  on  the  brass  merid- 
ian, at  the  degree  of  latitude  of  the  given  place,  and  lay  its 
edge  over  the  sun's  place  ;  then  count  the  number  of  degrees 
on  the  quadrant  between  this  point  and  the  wooden  horizon, 
and  it  will  give  the  altitude  required. 

The  distance  of  the  point,  where  the  edge  of  the  quadrant 
of  altitude  cuts  the  wooden  horizon,  from  the  north  or  south 
points,  will  give  the  sun's  azimuth. 

EXAMPLES. 

1.  Required  the  sun's  altitude,  &c.,  at  7  o'clock  in  the  morning  on 
the  5th  of  May  to  the  inhabitants  of  London. 

Answer.  Altitude  21^°,  and  azimuth  90°  from  the  north  point  of 
the  horizon. 

2.  Required  the  sun's,  altitude  and  azimuth  at  4  o'clock  in  the  after- 
noon on  the  2d  of  July,  to  the  inhabitants  of  Petersburg.    . 

3.  Required  the  same  as  in  the  last  example  to  the  inhabitants  of 
London. 

Answers.  (2.)  35°  altitude,  and  azimuth  75°  from  the  south  point; 
(3.)  37°  altitude,  and  azimuth  80°  from  the  south  point. 

PROBLEM  XVI.  The  hour  and  day  being  given  at  a  par- 
ticular place,  to  find  the  place  where  the  sun  is  then  vertical. 

•  RULE.  Find  the  sun's  declination  for  the  given  day ;  (see 
Prob.  XII. ;)  this  gives  the  latitude  of  the  required  place ; 


ON  THE  USE  OF  THE  GLOBES.  435 

bring  the  given  place  to  the  brass  meridian ;  set  the  index  to 
the  given  hour ;  turn  the  globe  till  the  index  points  to  XII. 
noon ;  then  all  the  places  under  the  brass  meridian  will  have 
noon  at  the  given  time,  and  the  place  whose  latitude  is  the 
same  as  the  sun's  declination  will  have  the  sun  vertical. 

EXAMPLES. 

>.   To  what  place  will  the  sun  be  nearly  vertical  on  the  5th  day  of 
February,  when  it  is  23  minutes  past  noon  at  London  ? 
Answer.    The  Island  of  St.  Helena. 

2.  To  what  place  will  the  sun  be  nearly  vertical  on  the  30th  day  of 
April,  when  it  is  34  minutes  past  1  o'clock'  in  the  afternoon  at  London  ? 

Answer.  '  To  the  Island  of  St.  Jago,  one  of  the  Cape  Verd  Isles. 

3.  When* it  is  40  minutes  past  6  o'clock  in  the  morning  at  London 
on  the  25th  of  April,  where  is  the  sun  vertical  ? 

Answer.     Madras. 

4.  When  it  is  4  o'clock  in  the  afternoon  at  London  on  the  18th  of 
August,  where  is  the  sun  vertical  ? 

Answer.    Barbadoes. 

PROBLEM  XVII.  A  place  within  the  torrid  zone  being 
given,  to  find  those  two  days  of  the  year  on  which  the  sun  will 
be  vertical  to  the  given  place.  9 

RULE.  Bring  the  giyen  place  to  the  brass  meridian,  and 
observe  its  latitude ;  turn  the  globe  on  its  axis,  and  mark  what 
two  points  of  the  ecliptic  pass  under  that  latitude ;  seek  those 
two  points  of  the  ecliptic  in  the  circle  containing  the  signs  of 
the  zodiac,  on  the  wooden  horizon,  and  opposite  to  them  will 
be  found  the  days  required. 

EXAMPLES. 

1.  On  what  two  days  of  the  year  will  the  sun  be  vertical  to  the  in- 
habitants of  St.  Helena  ? 

Answer.  On  the  5th  day  of  February,  and  on  the  6th  day  of  No- 
vember. 

2.  On  what  two  days  of  the  year  will  the  sun  be  vertical  to  the  inhab- 
itants of  Madras  ? 

Answer.   On  the  25th  day  of  April,  and  on  the  18th  of  August. 


436         NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

PROBLEM  XVIII.  The  hour  and  day  being  given  at  a 
particular  place,  to  find  the  places  where  the  sun  is  then  rising 
or  setting,  and  where  it  is  noon  or  midnight. 

.EULE.  Rectify  the  globe  for  the  latitude  of  the  given 
place ;  find  (by  Prob.  XVI.)  the  place  to  which  the  sun  is 
vertical  at  the  given  time,  and  bring  that  place  to  the  brass 
meridian ;  then  all  places  on  the  western  edge  of  the  wooden 
horizon  will  have  the  sun  rising ;  all  those  on  the  eastern  edge 
will  have  him  setting;  all  those  places  under  the  upper  half 
of  the  brass  meridian  will  have  noon  ;  and  all  those  under  the 
lower  half  of  the  brass  meridian  will  have  midnight. 

EXAMPLES. 

1.  When  it  is  52  minutes  past  4  o'clock  in  the  morning  at  London  on 
the  5th  of  March,  at  what  places  is  the  sun  then  rising,  or  setting,  and 
where  is  it  noon  or  midnight  ? 

Answer.  The  sun  is  rising  at  the  White  Sea,  Morea,  Petersburg,  &c. ; 
setting  at  the  eastern  coast  of  Kamtschatka,  between  the  Friendly  and 
Society  Islands,  &c. ;  noon  at  Sunda  Islands,  Cochin  China,  &c. ;  mid- 
night at  New  York,  St.  Domingo,  &c. 

2.  Wljere  is  the  sun  rising,  setting,  &c.,  when  it  is  4  o'clock  in  the 
afternoon  at  London  on  the  25th  day  of  April  ? 

Answer.  Rising  at  Hawaii,  &c. ;  setting  at  the  Cape  of  Good  Hope, 
&c. ;  noon  at  Buenos  Ayres,  &c. ;  and  so  on. 

PROBLEM  XIX.  To  illustrate  the  three  positions  of  the 
sphere,  right,  parallel,  and  oblique,  so  as  to  show  the  aspect  of 
the  sun,  fyc.,  at  different  times  of  the  year. 

1.  The  right  sphere.  The  people  at  the  equator  have  this 
sphere ;  the  north  po|ar  star  always  appears  in  their  horizon. 
To  place  the  artificial  globe  in  this  position,  bring  the  two 
poles  to  the  wooden  horizon ;  turn  the  globe  round ;  then  the 
following  facts  may  be  readily  illustrated  :  — 

At  the  equator,  the  days  are  always  twelve  hours  long, 
whatever  may  be  the  position  of  the  sun  in  the  ecliptic ;  for 
the  sun  an'd  all  the  heavenly  bodies  will  appear,  to  revolve 
round  the  earth  in  circles  parallel  to  the  equinoctial,  and  the 


ON  THE  USE  OF  THE  GLOBES.  437 

diurnal  'arc  above  the  horizon  will  always  be  equal  to  that 
which  is  below  it.  The  whole  of  the  heavens  may  be  seen  at 
the  equator  in  the  course  of  a  day ;  and  in  the  course  of  the 
year  all  the  stars  in  the  heavens  may  be  seen ;  whereas,  at 
the  poles,  only  one  half  of  the  heavens  can  be  seen.  On  the 
equinoxes  the  sun  passes  directly  over  the  heads  of.  the 
people  at  the  equator ;  when  the  sun  is  in  the  northern  half 
of  the  ecliptic,  at  noon  his  aspect  is  north  ;  and,  on  the  con- 
trary, when  the  sun  is  in  the  southern  half  of  the  ecliptic,  at 
noon  his  aspect  is  south. 

2.  TJie  parallel  sphere.  The  people  at  the  north  pole,  if 
there  were  any  living  there,  would  have  this  sphere;  the 
north  polar .  star  in  the  heavens  would  appear  exactly  over 
their  heads.  To  place  the  artificial  globe  in  this  position, 
elevate  the  north  pole  90°  above  the  horizon,  or,  what  is  the 
same  thing,  make  the  equinoctial  to  coincide  with  the  wooden 
horizon. 

At  the  poles,  during  six  months  of  the  year,  the  sun  shines 
without  setting,  and  during  the  other  six  months  he  never 
appears  above  the  horizon.  On  the  21st  day  of  March,  when 
the  sun  is  in  the  vernal  equinox,  he  will  be  seen  by  the  people 
at  the  north  pole  (if  there  were  any)  to  skim  along  the  hori- 
zon ;  and  as  the  sun  increases  in  his  northern  declination,  he 
will  appear,  day  after  day,  to  rise  higher  above  the  horizon, 
until  he  attains  his  greatest  northern  declination,  (23i°,)  and 
then  his  elevation  above  the  horizon  will  be  23<T,  that  is,  it 
will  be  equal  to  his  declination  ;  after  this,  he  will  gradually 
decrease  in  his  altitude,  until  he  arrives  at  the  autumnal  equi- 
nox, when  he  will  again  appear  to  skim  along  the  edge  of  the 
horizon ;  so  that  he  will  have  been  six  months  above  the 
horizon  without  setting ;  after  this  he  will  totally  disappear 
for  six  months.  But  there  will  be  twilight  until  the  sun  is 
18°  below  the  horizon,  —  that  is,  until  he  has  attained  18° 
south  declination.  The  same  thing  will  take  place  with 
respect  to  the  south  pole,  but  with  this  difference ;  while  the 
37* 


438  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

sun  shines  upon  the  north  pole,  he  will  be  invisible  to  the 
supposed  people  of  the  south  pole,  and  vice  versa. 

A  spectator  at  the  north  pole  can  only  see  the  stars  in  the 
northern  hemisphere,  or  those  stars  which  lie  on  the  north  of 
the  equinoctial. 

3.  The  oblique  sphere.  All  people  living  on  the  earth, 
excepting  those  at  the  equator  and  poles,  have  this  position  of 
the  sphere.  In  this  case,  the  horizon  cuts  the  equator  ob- 
liquely. To  place  the  artificial  globe  in  this  position,  elevate 
the  north  or  south  pole,  as  the  case  may  be,  to  the,  latitude  of 
the  place  where  we  may  conceive  a  spectator  to  be  placed. 
Let  us  suppose,  for  example,  .that  the  north  pole  is  elevated  to 
the  latitude  of  London. 

To  the  people  living  at  London,  for  six  months  of  the  year, 
the  days  are  more  than  twelve  hours  long,  and  for  the  re- 
maining *  six  months,  they  are  less  than  twelve  hours  long ; 
that  is  to  say,  from  the  21st  of  March  to  the  22d  of  Septem- 
ber, when  the  sun  is  on  the  northern  side  of  the  equinoctial, 
the  days  are  more  than  twelve  hours  long ;  and,  on  the  con- 
trary, from  the  22d  of  September  to  the  21st  of  March,  when 
the  sun  is  on  the  southern  side  of  the  equinoctial,  the  days 
are  less  than  twelve  hours  long.  At  the  vernal  equinpx  (on 
the  21st  of  March)  the  sun  shines  perpendicularly  over  the 
equator,  and  the  days  and  nights  are  equal  all  over  the  'globe ; 
as  the  sun  increases  in  his  northern  declination,  the  days  also 
increase  in  length ;  for  the  diurnal  arcs  described  by  the  sun 
are  unequally  divided  by  the  horizon  ;  when  the  sun  has 
attained  his  greatest  northern  declination,  (June  21st,)  the 
days  have  also  attained  their  greatest  length  ;  but  they  will 
be  at  their  shortest  to  the  people  in  the  southern  hemisphere  ; 
after  this,  the  sun's  northern  declination  gradually  decreases, 
and  the  days  also  gradually  decrease  in  length ;  when  he 
arrives  at  the  autumnal  equinox,  (Sept.  22d,)  the  days  and 
nights  are  again  equal ;  after  this,  the  days  become-  shorter 
and  shorter,  as  the  sun's  southern  declination  increases,  until 
he  has  attained  his  greatest  southern  declination,  (December 


ON   THE    USE    OF   THE    GLOBES.  439 

21st,)  and  then  the  days  will  be  at  their  shortest  with  us,  but 
at  their  greatest  length  to  the  people  of  the  southern  hemi- 
sphere ;  after  this,  our  days  increase  in  length,  and  when  the 
sun  again  arrives  at  the  vernal  equinox,  the  days  and  nights 
are  again  equal. 

The  duration  of  twilight  is  greater  with  ns  than  it  is  at  the 
equator,  because  the  diurnal  arc  of  the  sun  cuts  the  horizon 
obliquely,  which  causes  him  to  take  a  longer  time  to  get  18° 
below  the  horizon  ;  whereas,  at  the  equator,  the  sun  sinks 
perpendicularly  below  the  horizon,  which  tends  to  shorten  the 
duration  of  twilight. 

The  people  that  live  in  the  northern  hemisphere  can  never 
see  those  stars  which  lie  towards  the  south  polar  star,  and  the 
people  in  the  southern  hemisphere  can  never  see  those  stars 
which  lie  towards  the  north  polar  star  ;  but,  as  already  ob- 
served, a  person  at  the  equator  may  see  all  the  stars  in  the 
heavens  in  the  course  of  the  year. 

PROBLEM  XX.  Any  place  in  the  north  frigid  zone  being 
given,  to  find  how  long  the  sun  shines  there  without  setting, 
and  how  long  he  is  invisible. 


Rectify  the  globe  to  the  latitude  of  the  place  ; 
bring  the  ascending  signs  of  the  ecliptic  (the  signs  going  be- 
fore Cancer)  to  the  north  point  of  the  horizon,  and  observe 
what  degree  of  the  ecliptic  is  cut  by  that  point  ;  find  on  the 
wooden  horizon  the  day  and  month  '  corresponding  to  that 
degree  ;  then  from  that  day  the  sun  begins  to  shine  without 
setting.'  Now,  bring  the  descending  signs  (the  signs  coming 
after  Cancer)  to  the  north  point  of  the  horizon,  and  observe 
what  degree  of  the  ecliptic  is  cut  by  that  point  ;  find  on  the 
wooden  horizon,  as  before,  the  day  and  month  corresponding 
to  that  degree  ;  then,  on  that  day,  the  sun  ceases  to  shkie 
without  setting.  "  By  proceeding  in  the  same  manner  with  the 
southern  point  of  the  horizon,  we  may  find  the  beginning  and 
end  of  the  period  during  which  the  sun  is  invisible. 


440  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Example,-  How  long  will  the  sun  shine  without  setting  to  the  inhab- 
itants of  the  North  Cape,  in  latitude  7l£°  north  ? 

Answer.  The  sun  begins  to  shine  continually  on  the  14th  of  May, 
and  ceases  to  shine  continually  on  the  30th  of  July.  The  longest  day 
is,  therefore,  77  days  long  ;  that  is  to  say,  the  sun  shines  without  setting 
for-  77  days.  The  period  during  which  the  sun  will  be  invisible  extends 
from  the  16th  of  November  to  the  27th  of  January.  The  longest  night 
is,  therefore,  73  days  long ;  that  is  to  say,  the  sun  is  never  seen  by  the 
inhabitants  of  this  place  for  the  period  of  73  days. 

PROBLEM  XXL  To  find  the'  beginning  and  end  of  twi- 
light at  a  given  place  on  any  given  day. 

RULE.  Rectify  the  globe  for  the  latitude  of  the  place ; 
bring  the  sun's  place  in  the  ecliptic-  on  the  given  day  to  the 
brass  meridian  ;  set  the  hour  circle  to  XII. ;  screw  the  quad- 
rant of  altitude  upon  the  brass  meridian  over  the  given  lati- 
tude ;  turn  the  globe  westward  till  the  sun's  place-  conies  to 
the  western  edge  of  the  wooden  horizon ;  then  the  hour  circle 
will  show  the  time  of  the  sun's  setting,  or  the  beginning  of  * 
evening  twilight ;  continue  the  motion  of  the  globe  till  the 
sun's  place  coincides  with  18  degrees  on  the  quadrant  of  alti- 
itude,  below  the  horizon ;  then  the  hour  circle  will  show  the 
time  at  which  the  evening  twilight  ends.  The  duration  of 
twilight  is  equal  to  the  difference  between  the  time  at  which 
it  en^s  and  the  time  at  which  it  begins.  The  time  at  which 
evening  twilight  ends,  subtracted  from  12  will  give  the  begin- 
ning of  morning  twilight,  which  is  of  the  same  duration  as 
the  evening  twilight. 

..  EXAMPLES. 

1.  Required  the  duration  of  twilight  at  London  on  the  22d  of  Sep- 
tember. . 

Answer.  The  suti  sets  at  6  o'clock,  and  twilight  ends  at  8  o'clock; 
consequently  the  duration  of  twilight  is  2  hours. 

2.  Required  the  duration  of  twilight  at  those  places  which  have  the 
same  latitude  as  Edinburgh,  on  the  24th  of  April. 

Answer.     3  hours. 

3.  What  is  the  duration  of  twilight  at  London  on  the  20th  of  April  ? 
Answer.     2  hours  18  minutes. 


ON    THE    USE    OF   THE    GLOBES.  441 

PROBLEM  XXII.  Given  the  sun's  meridiem  altitude,  and 
the  day  of  the  month,  to  find  the  latitude  of  the  place. 

RULE,  If  the  sun  was  south  of  the  observer  when  the  alti- 
tude was  taken,  bring  the  sun's  place  in  the  ecliptic  to  the 
south  side  of  the  brass  meridian ;  move  the  brass  meridian  till 
the  sun's  place  is  raised  above  the  horizon  equal  to  the  given 
meridian  altitude ;  then  the  elevation  of  the  north  pole  will 
give  the  latitude  of  the  place.  If  the  sun  was  north  of  the 
observer  when  the  altitude  was  taken,  proceed  in  the  same 
manner,  with  this  exception,  that  the  sun's  place  must  be 
brought  to  the  north  side  of  the  brass  meridian,  and  the  eleva- 
tion of  the  south  pole  will  give  the  latitude  of  the  place. 

EXAMPLES. 

1.  On  the  21st  of  June,  the  meridian  altitude  of  the  sun  was  observed 
to  be  69 £°,  and  south  of  the  observer ;  required  the  latitude  of  the  place. 

Answer.     44°  north  latitude. 

2.  On  the  21st  of  December  the  meridian  altitude  of  the  sun  was  ob- 
served to  be  25°,  and  south  of  the  observer;  required  the  latitude  of  the 
place. 

Answer.     4l£°  north  latitude. 

3.  On  the  10th  of  May  the  meridian  altitude  of  the  sun  was  observed 
to  be  30°,  and  north  of  the  observer;  required  the  latitude  of  the  place. 

Answer.     42°  25'  south  latitude. 

* 

PROBLEM  XXIII.  To  find,  the  angle  of  position  between 
two  given  places. 

RULE.  If  the  two  places  be  on  -the  same  meridian,  they 
bear  north  and  south  from  each  other,  and  therefore  their  an- 
gle of  position  is  0.  When  the  places  are  not  on  the  same 
meridian,  proceed  as  follows :  rectify  the  globe  to  the  latitude 
of  one  of  the  places;  bring  that  place  to  the  brass  meridian, 
and  screw  the  quadrant  of  altitude  over  it ;  move  the  .quad- 
rant till  its  edge  falls  upon  the  other  place;  then  the  point 
where  the  edge  of  the  quadrant  cuts  the  wooden  horizon  will 
give  the  angle  of  position  between  the  two  .places,  which  is  es- 
timated in  degrees  from  the  north  point,  or  it  may  be  reckoned 
by  the  points  of  the  compass. 


442  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

EXAMPLES. 

1.  Required  the  angle  of  position  between  London  and  Madras. 
Answer.     90°  from  the  north  towards  the  east. 

2.  Required  the  angle  of  position  between  London  and  Jamaica. 
Answer.     The  quadrant  of  altitude  falls  upon  the  west  point  of  the 

horizon ;  the  angle  of  position  is  90°  from  the  north  towards  the  west. 

3.  What  is  the  angle  of  position  between  Madrid  and  Philadelphia  ? 
Answer.     60°. 

PROBLEM  XXIV.  To  find  all  the  places  to  which  a  lunar 
eclipse  is  visible  at  a  given  instant. 

RULE.  Find  (by  Prob.  XVI.)  the  place  to  which  the  sun 
is  vertical  at  the  given  time  ;  bring  the  place  to  the  brass  me- 
Tidian,  and  rectify  the  globe  to  the  latitude  of  that  place ;  then 
at  all  places  within  70  degrees  of.  this  place  an  eclipse  of  the 
sun  may  be  visible,  especially  if  it  be  a  total  eclipse.  For  a 
lunar  ellipse,  after  proceeding  as  before,  set  the  hour  circle  to, 
XII.  noon ;  turn  the  globe  till  the  hour  circle  is  at  XII.  mid- 
night ;  then  an  eclipse  of  the  moon  will  be  visible  to  all  those 
places  which  are  above  the  wooden  horizon. 

EXAMPLES. 

1.  There  was  an  eclipse  of  the  sun  on  the  9th  of  October,  1847,  at  29 
minutes  past  7  o'clock  in  the  morning,  at  London ;  to  what  places  might 
it  be  visible?  '* 

Answer.    To  Hindostan,  Arabia,  &c. 

2.  An  eclipse  of.  the  moon  took  place  on  the  26th  of  January,  1842, 
at  6  o'clock  in  the  afternoon,  at  London;,  to  what  places  was  it  visible? 

Answer.     Europe,  Asia,  Australia,  and  a  portion  of  Africa, 

3.  An  eclipse  of  the  moon  took  place  on  the  31st  of  May,  1844,  at  50 
minutes  past  10  in  the  evening,  at  London ;  to  what  places  was  it  visible  ? 

Answer.     Europe,  Africa,  and  a  portion  of  Asia. 

4.  An  eclipse  of  the  moon  will  take  place  on  the  7th  of  January,  1852, 
at  30  minutes  past  6  in  the  morning,  at  London ;  to  what  places  will  it 
be  visible  ? 

Answer.    Visible  at  London,  &c. 

PROBLEM  XXV.  To  place  the  terrestrial  globe  in  the  sun- 
shine, so  that  it  may  represent  the  actual  position  of  the  earth 
with  respect  to  the  sun. 


ON   THE    USE    OF   THE    GLOBES.  443 

KULE.  Place  the  globe  directly  north  and  south,  by  means 
of  the  mariner's  compass  usually  placed  beneath  the  globe, 
taking  care  to  bring  the  north  pole  of  the  needle  24  degrees 
to  the  west  of  the  north  point  of  the  compass,  which  is  the 
allowance  at  present  for  the  variation ;  *  bring  the  place  where 
you  are  living  to  the  brass  meridian,  and  elevate  the  pole  to 
its  latitude ;  then  the  globe,  with  its  various  lines,  &c.,  will 
correspond  in  every  respect  with  the  position  of  the  earth,  and 
the  imaginary  lines,  &c.,  upon  it,,  with  respect  to  the  sun. 
The  point  to  which  the  sun  is  vertical,  the  illuminated  hemi- 
sphere, &c.,  may  all  be  at  once  determined. 

PROBLEM  XXVI.  To  construct  a  horizontal  dial  by  the 
globe  for  a  given  latitude. 

RULE.  Place  the  globe,  as  in  the  last  problem,  directly 
north  and  south ;  rectify  the  globe  to  the  latitude  of  the  place ; 
bring  the  first  meridian  to  the  brass  meridian ;  then  observe 
the  points  where  the  hour  meridians  on  the  globe  cut  the 
horizon,  and  number  these  points  according  to  the  hours  of 
the  day ;  thus  the  point  of  the  dial. at  the  brass  meridian  must 
be  numbered  XII.,  thence  XL,  X.,  &c.,  towards  the  west  for 
the  morning  hours,  and  I.,  II.,  &c.,  for  the  evening  hours. 
The  style  of  the  dial  represents  the  axis  of  the  earth,  and 
must  therefore  always  make,  with  the  plane  of  the  horizon, 
or  the  plane  of  the  dial  plate,  an  angle  equal  to  the  latitude 

of  the  place. 

• 
• 

THE  CELESTIAL  GLOBE. 

DEFINITIONS   AND    EXPLANATIONS. 

1.  ^The  celestial  globe  is  constructed  to  represent  the  aspect 
of  the  heavens ;  all  the  stars  are  laid  down  on  its  surface  ac- 
cording to  their  relative  situations ;  and  the  various  imaginary 
circles  and  points  upon  the  .terrestrial  globe  are  supposed  to 

*  At  London. 


444          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

be  transferred  to  the  celestial  one.  The  rotatory  motion  of  this 
globe,  from  east  to  west,  represents  the  apparent  diurnal 
motion  of  the  sun,  moon,  and  stars,  to  a  spectator  supposed  to 
be  situated  in  the  centre  of  the  globe. 

2.  The  latitude  and  longitude  of  a  star  or  planet.  —  The 
latitude  of  a  body,  on  the  celestial  globe,  is  its  distance  from 
the  ecliptic,  north  or  south,  measured  in  degrees  on  a  great 
circle  passing  through  the  body  and  the  pole  of  the  ecliptic ; 
and  the  longitude  is  the  distance  of  the  point,  where  the  great 
circle  cuts  the  ecliptic,  from  the  first  point  of  Aries.    '  Latitude 
and  longitude  are  referred  to  the  ecliptic,  on  the  celestial 
globe,  but  on  the  terrestrial  globe  they  are  referred  to  the 
equator. 

3.  The  declination  and  right  ascension  of  a  heavenly  body.  — 
The  declination  of  a  body  is  its  distance  from  the  equinoctial, 
north  or  south,  measured  in  degrees  on  a  meridian  passing 
through  the  body ;  and  the  right  ascension  is  the  distance  of 
the  point  where  this  meridian  cuts  the  equinoctial,  from  the 
first  point  of  Aries.     The  right  ascension  of  a  body  is  some- 
times expressed  in  hours,  making  the  usual  allowance  of  one 
hour  of  time  for  15  degrees  of  distance. 


PROBLEMS  ON  THE  CELESTIAL  GLOBE. 

PROBLEM  I.  To  find  the  right  ascension  and  declination 
.  of  the  sun  or  of  a^star. 

RULE.  Bring  the  sun's  .place,  or  the  given  star,  to  the 
brass  meridian ;  the  degree  over  it  is  the  declination,  and '  the 
degree  on  the  equator  cut  by  the  brass  meridian  gives  the 
right  ascension. 

EXAMPLES.  0 

1.  Required  the  right  ascension  and  declination  of  Regulus,  in  the 
constellation  of  the  Lion. 

Answer.    Right  ascension  150°,  declination  12°  47'  north. 
Required  the  right  ascension  and  declination  of  the  following  stars :  — 


ON   THE    USE    OF   THE    GLOBES.  445 

2.  Capella,  in  the  constellation  of  Auriga ;  3.  Dubhe,  in  the  constel- 
lation of  the  Great  Bear ;  4.  Aldebaran,  in  the  constellation  of  Taurus ; 
5.  Arcturus,  in  the  constellation  of  Bootes. 

Answers.     (2.)   Right  ascension  76°,  declination  45°  49'  N. 

(3.)   Right  ascension  163°  15',  declination  62°  36'  N. 

(4.)   Right  ascension  66°,  declination  16°  10'  ft. 

(5.)   Right  ascension  212°,  declination  20°  3'  N. 

PROBLEM  II.  TJie  right  ascension  and  declination  of  a 
heavenly  body  being  given,  to  find  its  place  on  the  globe. 

RULE.  Bring  the  given  degree  of  right  ascension  (or  the 
given  time  of  right  ascension)  to  the  brass  meridian;  then 
under  the  given  degrees  of  declination,  reckoned  on  the  brass 
meridian,  you  will  find  the  place  of  the  body. 

EXAMPLES. 

1.  Required  the  star  whose  right  ascension  is  76°  45',  or  5  hours  7 
minutes,  and  declination  8°  24'  south. 

Answer.  Rigel,  a  star  of  the  first  magnitude  in  the  constellation  of 
Orion. 

What  stars  have  the  following  right  ascensions  and  declinations  ? 

Right  Ascensions.  Declinations. 

2.  261°  30'  or  17  h.  26  m.  52°  25'  N. 

3.  6  h.  38  m.  16°  29'  S. 

4.  19  h.  43  m.  8°  26'  N. 

5.  7  h.  35  m.  28°  26'  N. 

Answers.  (2.)  /?,  a  star  of  the  second  magnitude  in  the  constellation 
of  Draco ;  (3.)  Sirius,  in  the  Great  Dog ;  (4.)  Altair,  in  the  Eagle ; 
(5.)  Pollux,  the  south  twin. 

PROBLEM.  III.  To  find  the  latitude  and  longitude  of  any 
star. 

RULE.  Bring  the  pole  of  the  ecliptic  to  the  brass  merid- 
ian ;  fix  the  quadrant  of  altitude  over  the  pole,  and  move  the 
quadrant  till  its  edge  comes  over  the  star ;  then  the  degree  of 
the  quadrant  over  the  star  is  the  latitude,  and  the  number  of 
degrees  between  the  edge  of  the  quadrant  and  the  first  point 
of  Aries  is  the  longitude. 
38 


446  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

EXAMPLES. 

1.  What  is  the  latitude  and  longitude  of  Aldebaran,  in  the  constella- 
tion of  Taurus  ? 

Answer.     Latitude  5°  28'  S.,  longitude  2  signs  6°  53'. 

2.  What  is  the  latitude  and  longitude  of  Pollux,  in  the  constellation 
of  Gemini  ? 

Answer.     Lat.  6°  30'  N.,  long.  3  signs  21°. 

PROBLEM  IV.  The  day  and  hour,  and  the  latitude  of  the 
place,  being  given,  to  place  the  celestial  globe  so  as  to  represent 
the  appearance  of  the  heavens  at  that  place  and  time. 

RULE.  Place  the  globe  north  and  south,  by  the  mariner's 
compass  ;  rectify  the  globe  to  the  latitude  of  the  place  ;  bring 
the  sun's  place  in  the  ecliptic  to  the  brass  meridian;  set  the 
hour  circle  to  XII. ;  turn  the  globe  till  the  index  of  the  hour 
circle  points  to  the  given  hour  of  .the  day ;  then  in  this  posi- 
tion the  stars  figured  on  the  globe  will  exactly  correspond  with 
the  actual  appearance  of  the  stars  in  the  heavens. 

PROBLEM  V.  The  day  and  hour,  and  the  latitude  of  the 
place,  being  given,  to  find  what  stars  -are  rising,  setting,  and 
culminating. 

RULE.  Rectify  the  globe  for  the  latitude  of  the  place ; 
bring  the  sun's  place  to  the  brass  meridian ;  put  the  hour  cir- 
cle to  XII. ;  turn  the  globe  till  the  hour  circle  indicates  the 
given  hour  of  the  day ;  then  all  the  stars  on  the  eastern  semi- 
circle will  be  rising,  those  on  the  western  semicircle  will  be 
setting,  those  under  the  brass  meridian  will  be  culminating,  or 
in  their  southing,  and  those  stars  above  the  wooden  horizon 
will  be  visible  at  the  given  time  and  place. 

To  determine  those  stars  which  never  set,  turn  the  globe 
on  its  axis  ;  then  those  stars  which  do  not  go  below  the  wood- 
en horizon  never  set  at  the  given  place. 

EXAMPLES. 

1 .  To  find  the  constellations  which  are  rising,  setting,  and  culminating, 
on  the  20th  of  January,  at  2  o'clock  in  the  morning  at  London. 


ON   THE   USE    OF   THE    GLOBES.  447 

Answer.  The  constellation  of  Lyra,  &c.,  are  rising;  Andromeda, 
&c.,  are  setting ;  and  the  Great  Bear,  &c.,  are  on  the  meridian. 

2.  To  find  the  stars  which  are  rising,  setting,  and  culminating,  on  the 
8th  of  February,  at  9  o'clock  in  the  evening  at  London. 

Answer.     A  star  in  the  Northern   Crown  is  rising ;    Arcturus,  in 
Bootes,  is  a  little  above  the  horizon  ;  Sirius  is  on  the  meridian ;  Markab, 
in  Pegasus,  a  little  below  the  western  horizon. 
• 

PROBLEM  VI.  To  find  the  time  when  any  heavenly  body 
will  rise)  come  to  the  meridian,  and  set,  on  a  particular  day, 
at  any  given  place. 

RULE.  Rectify  the  globe  for  the  latitude  of  the  place ; 
bring  the  sun's  place  in  the  ecliptic  to  the  brass  meridian ;  set 
the  hour  circle  to  XII. ;  turn  the  globe  till  the  given  star  * 
comes  to  the  eastern  edge  of  the  wooden  horizon ;  then  the 
hour  circle  will  show  the  time  of  rising ;  now  turn  the  globe 
till  the  star  comes  to  the  brass  meridian,  and  the  hour  circle 
will  show  the  time  of  its  culmination  or  southing ;  lastly,  turn 
the  globe  till  the  star  comes  to  the  western  edge  of  the  wooden 
horizon,  and  the  hour  circle  will  show  the  time  of  setting. 

EXAMPLES. 

1.  At  what  time  will  Arcturus,  in  the  constellation  of  Bootes,  rise, 
culminate,  and  set,  at  London  on  the  7th  of  September  ? 

Anstoer.  Arcturus  will  rise  at  about  a  quarter  past  7  in  the  morning, 
culminate  at  a  quarter  past  3  in  the  afternoon,  and  set  at  three  quarters 
past  10  at  night. 

2.  At  what  time  will  Aldebaran,  in  the  constellation  of  Taurus,  rise, 
&c.,  at  Edinburgh  on  the  26th  of  November  ? 

Answer.    It  rises  at  about  half  past  4  in  the  afternoon,  &c. 

PROBLEM  VII.  The  day  of  the  month,  the  latitude  of  the 
place,  and  the  altitude  of  a  star  being  given,  to  find  the  hour 
of  the  night. 

RULE.  Rectify  the  globe  for  the  latitude  of  the  place ; 
bring  the  sun's  place  in  the  ecliptic  to  the  brass  meridian ;  set 

*  The  place  of  a  planet  on  the  globe  must  be  found  by  Prob.  VIII. 


448          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

the  hour  circle  to  XII. ;  screw  the  quadrant  of  altitude  to  the 
zenith,  and  turn  it  to  that  side  of  the  meridian  on  which  the 
star  was  observed ;  move  the  globe  and  the  quadrant  till  the 
star  is  on  the  degree  of  the  quadrant  equal  to  the  given  alti- 
tude ;  then  the  hour  circle  will  show  the  hour  required. 

EXAMPLES.  • 

1.  At  Rome  on  the  2d  of  December,  the  star  Capella,  in  the  constella- 
tion of  Auriga,  was  observed  to  be  42°  above  the  horizon,  and  west  of 
the  meridian ;  required  the  hour. 

Answer.    Five  o'clock  in  the  morning. 

2.  At  London  on  the  29th  of  December,  the  star  Deneb,  in  the  tail 
of  the  Lion,  was  found  to  be  40°  above  the  horizon,  and  east  of  the 
meridian ;  required  the  hour. 

Answer.     About  a  quarter  past  2  o'clock  in  the  morning. 

PROBLEM  VIII.  Given  the  year  and  the  day,  to  find  the 
place  of  a  planet  on  the  globe. 

RULE.  Bring  the  sun's  place  in  the  ecliptic  to  the  brass 
meridian ;  set  the  hour  circle  to  XII. ;  find,  in  the  Nautical 
Almanac,  the  time  when  the  planet  passes  the  meridian  on  the 
given  day,  and  turn  the  globe  till  the  index  of  the  hour  circle 
points  to  the  hour  thus  found ;  find,  in  the  Almanac,  the  dec- 
lination of  the  planet  for  the  same  day ;  then  under  this  decli- 
nation, found  on  the  brass  meridian,  is  the  place  of  the  planet. 


EXPERIMENTAL  CHEMISTRY. 


SECTION  I. 

NATURE    OF   CHEMISTRY.      SIMPLE   AND    COMPOUND    BODIES. 
ATTRACTION.       CHEMICAL    AFFINITY.        NATURE    OF   ACIDS 

AND    ALKALIES.      SOLUTIONS. 

• 

NATURE    OF    CHEMISTRY.       SIMPLE    AND    COMPOUND    BODIES. 

1.  CHEMISTRY  is  that  science  which  treats  of  the  proper- 
ties of  the  simple  substances  composing  the  globe,  and  of  the 
various  compounds  resulting  from  their  action  upon  each 
other.  So  far  as  our  present  knowledge  extends,  there  are 
sixty-two  simple  or  elementary  substances,  which,  uniting 
with  each  other,  form  the  vast  variety  of  substances  found  in 
the  earth,  the  air,  and  the  waters  of  the  ocean  and  rivers.  A 
simple  substance,  do  with  it  what  we  may,  will  not  yield  any 
other  kind  of  substance  different  from  itself.  Thus  iron  is 
considered  to  be  a  simple  body,  because  we  can  only  obtain 
iron  from  it.  A  compound  body  contains  two  or  more  simple 
substances  in  a  state  of  chemical  combina- 
tion. Nearly  all  the  substances  in  nature 
are  compounds.  Sulphur  and  iron  are 
simple  substances,  but  they  combine  and 
form  a  compound  substance  called  sul- 
phuret  of  iron. 

Experiment.  Take  some  iron  filings  pid  mix 
them  intimately  with  about  half  their  weight  of 
sulphur ;  put  the  mixture  into  a  test  tube,  and 
apply  the  flame  of  a  spirit  lamp ;  at  the^ame  time  close  the  mouth  of 
the  tube  with  the  fore  finger,  to  exclude  the  air  :  the  iron  and  sulphur 
38*  (449) 


450  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

combine  with  ignition,  forming  the  compound  of  sulphuret  of  iron  —  a 
black  substance   entirely  different   from  either  the  iron  or  sulphur. 

2.  Elementary   substances    are   usually  divided   into   two 
classes,  namely,  metallic  and  non-metallic.     The  following  list 
comprises  some  of  the  most  important  elementary  substances : — 

Non-metallic  Elements. 

Oxygen,    >       ^  foun(j  jn  tne  atmosphere ; 

Nitrogen,  > 

Hydrogen,  Chlorine,  Carbon,  Sulphur,  Phosphorus,  Iodine,  &c. 

Metals. 

Potassium,  the  metal  which  forms  potassa  by  combining  with  oxygen ; 
Sodium,  the  metal  which  forms  soda ; 
Calcium,  the  metal  which  forms  lime ; 
Magnesium,  the  metal  which  forms  magnesia ; 

Iron,  Copper,  Zinc,  Tin,  Lead,  Manganese,  Arsenic,  Chromium,  Mercury, 
Silver,  Gold,  Platinum,  &c. 

3.  There  are  many  substances,  which,  although  they  appear 
simple,  are  in  reality  of  a  compound  nature.     Thus  water  is 
a  compound,  being  made  up  or  composed  of  oxygen  and  hy- 
drogen ;  the  air  is  chiefly  a  mixture  of  oxygen  and  nitrogen ; 
common  salt  is  a  compound,  containing  chlorine  and  sodium ; 
and  so  on  to  other  cases. 

DIFFERENT   KINDS    OF    ATTRACTION. 

4.  Attraction  is  one  of  the  distinguishing  qualities  of  ma- 
terial substances.     There  are  various  kinds  of  attraction. 

Attraction  of  gravitation.  —  A  stone  falls  to  the  ground  in 
consequence  of  the  earth's  attraction,  and  the  planets  in  the 
solar  system  are  maintained  in  their  orbits  round  the  sun  by 
the  attractive  force  which  he  exerts  upon  them.  This  is  called 
the  attraction  of  gravitation,  and  it  subsists  between  bodies  at 
all  definite  distances  from  each  other. 

5.  Magnetic  attraction.  —  This  is  familiarly  exhibited  in  the 
attraction  which  the  poles  of  a  magnet  have  for  soft  iron. 

6.  Electrical  attraction. 

Experiment.     If  a  stick  of  sealing  wax  (or  a  glass  tube)  be  rubbed 


EXPERIMENTAL    CHEMISTRY.  45 1 

sharply  with  a  dry  silk  handkerchief,  the  sealing  wax  will  attract  small 
cuttings  of  light  paper.     This  is  called  electrical  attraction. 

7.  Attraction  of  cohesion. 

Exp.  1.  If  an  apple  be  cut  in  two  with  a  sharp  knife,  the  pieces  may 
be  put  together  so  as  to  adhere. 

Exp.  2.  Take  two  balls  of  lead  ;  scrape  a  clean  portion  in  each  ;  bring 
the  clean  parts  in  contact,  and  rub  the  balls  together  by  giving  them  a 
circular  motion  :  they  stick  or  cohere  together. 

Exp.  3.  Two  polished  plates  of  metal  placed  together  require  consid- 
erable force  to  separate  them. 

The  force  manifested  in  these  experiments  is  called  the 
attraction  of  cohesion,  or  adhesion.  The  minute  particles,  or 
molecules,  of  which  bodies  are  composed,  are  held  together 
by  the  attraction  of  cohesion  subsisting  amongst  these  parti- 
cles. Bodies  are  solid,  liquid,  or  aeriform,  according  as  the 
force  of  cohesion  is  modified  by  heat. 

8.  Capillary  attraction  is  a  peculiar  form  of  cohesion. 
Exp.  1 .   Plunge  the  extremity  of  a  small  glass  tube  in  water  :  the  fluid 

rises  within  the  small  bore  of  the  tube. 

Exp.  2.  Place  a  piece  of  lump  sugar  on  a  few  drops  of  water :  the 
fluid  rises  through  the  fine  pores  of  the  sugar. 

CHEMICAL    ATTRACTION,    OR    AFFINITY. 

9.  However  intimately  the  sulphur  and  iron,  in  the  experi- 
ment Art.  1  may  be  mixed,  we  can  only  by  this  means  pro- 
duce a  mechanical  mixture  of  the  particles  of  the  two  sub- 
stances ;  but,  after  chemical  combination,   there  is  no  trace 
left  of  either  the  sulphur  or  the  iron.      Chemical  affinity  dif- 
fers, in  certain  respects,  from  all  other  kinds  of  attraction.     It 
resembles  cohesion,  inasmuch  as  it  subsists  between  the  par- 
ticles, of  matter  and  holds  them  together;  but  while  cohesion 
takes  place  between  particles  of  the  same  sort,  affinity  is  ex- 
erted between  the  particles  of  different  kinds  of  matter ;  and 
while  cohesion  produces  no  change  in  the  properties  of  a  sub- 
stance, affinity  is  almost  invariably  attended  with  a  marked 
change  in  the  appearance  and  other  properties  of  the  sub- 
stances forming  the  compound.     All  chemical  changes   are 


452  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

produced  by  affinity,  or  chemical  attraction.  Combination  and 
decomposition  are  the  results  of  chemical  action. 

Combination  takes  place  when  particles  of  different  kinds 
of  matter  unite  and  form  a  new  substance. 

Decomposition  takes  place  when  a  substance  is  resolved  into 
the  different  kinds  of  matter  of  which  it  is  composed  or  made  up. 

EXPERIMENTS. 

1.  To  a  glass  of  water  add  a  little  oil :  the  oil  floats  upon  the  water, 
but  does  not  combine  with  it.     Water,  therefore,  has  no  affinity  for  oil. 

2.  Add  ammonia  ;  stir  the  mixture  with  a  glass  rod  :  the  oil  and  the 
ammonia  combine,  and  form  a  soapy  substance,  called  a  liniment.     Oil 
and  ammonia,  therefore,  have  an  affinity  for  each  other.     This  is  a  case 
of  simple  combination. 

3.  To  the  soapy  compound  in  the  last  experiment  add  a  few  drops  of 
sulphuric  acid,  (oil  of  vitriol ;)  the  ammonia,  having  a  greater  affinity 
for  the  sulphuric  acid,  quits  the  oil,  and  combines  with  the  acid,  forming 
the  sulphate  of  ammonia  :  the  oil,  being  set  free,  again  floats  upon  the 
surface.     This  is  a  case  of  composition  as  well  as  of  decomposition  :  it  is 
therefore  an  instance  of  what  is  called  single  elective  affinity. 

4.  Dissolve  some  acetate  of  lead  (sugar  of  lead)  in  a  glass  of  water ;  * 
add  a  few  drops  of  sulphuric  acid :  a  white  compound  of  sulphuric  acid 
and  oxide  of  lead  is  precipitated,  or  falls  to  the  bottom  of  the  glass. 
This  is  also  a  case  of  single  elective  affinity. 

5.  To  a  solution  of'  acetate  of  lead,  add  a  few  drops  of  a  solution  of 
sulphate  of  soda,  (Glauber  salts :)  sulphate  of  lead  is  precipitated,  as  in 
the  last  experiment,  and  acetate  of  soda  remains  in  solution.     Here  there 
is  a  mutual  interchange  of  substances  :  hence  it  is  called  a  case  of  double 
elective  affinity. 

10.  Compositions,  as  well  as  decompositions,  are  continually 
going  on  in  the  processes  of  art  and  nature.  A  piece  of  chalk, 
(carbonate  of  lime,)  heated  to  redness  in  the  fire,  gives  off  a 
substance  called  carbonic  acid  gas,  and  quick  lime  is  left. 
When  charcoal  (the  carbon  obtained  from  wood)  is  burned 
away,  the  oxygen  in  the  air  combines  with  the  carbon  or  char- 
coal, and  forms  carbonic  acid  gas,  which  is  of  course  thrown 
into  the  air,  and  is  thus  apparently  lost ;  but  there  is  no  such 
thing  as  destruction  or  annihilation  in  nature,  for  substances 

*  When  any  substance  is  dissolved  in  water,  it  is  called  a  solution  of  that 
substance. 


EXPERIMENTAL    CHEMISTRY.  453 

can  only  change  their  form  of  combination.  When  a  piece 
of  lump  sugar  is  dissolved  in  water,  the  sugar,  although  no 
longer  visible,  is  not  destroyed;  it  has  combined  with  the 
water,  forming  a  solution  of  sugar.  In  like  manner,  we  are 
able  to  explain  all  other  changes  of  form  which  bodies  un- 
dergo around  us. 

11.  NATURE  OF  ACIDS  AND  ALKALIES. 

EXPERIMENTS. 

1.  Add  a  few  drops  of  sulphuric  acid  to  a  glass  of  water;  taste  the 
diluted  acid :  it  is  sour  or  acid  to  the  taste.     Add  a  little  of  the  vegetable 
blue  liquor  of  red  cabbage  *  to  a  glass  of  water ;  add  a  little  of  the  diluted 
sulphuric  acid  to  this  blue  solution :  it  is  changed  to  a  red  color.     The 
same  experiment  may  be  performed  with  any  other  acid. 

Thus  acids  are  sour  to  the  taste,  and  change  vegetable  blue 
colors  to  red. 

2.  Ammonia,  potassa,  and  soda  are  the  most  common'  and  important 
alkalies.     Add  drop  by  drop  of  a  solution  of  ammonia  to  the  red  liquor 
of  the  last  experiment,  until  the  red  color  is  changed  to  a  greenish  blue. 
Taste  the  liquid :  it  is  no  longer  sour  or  acid.     Add  now  more  acid,  drop 
by  drop,  until  the  red  color  is  restored ;  and  so  on. 

Thus  alkalies  neutralize  the  effect  of  acids,  and  change  the 
vegetable  blues  to  green. 

Blue  slips  of  paper,  stained  by  litmus,f  are  commonly  used  to  ascertain 
when  an  alkali  exactly  neutralizes  an  acid. 

3.  To  liquid  ammonia  add  sulphuric  acid,  until  a  slip  of  blue  litmus 
paper,  dipped  into  the  mixture,  is  about  to  change  its  color  to  red.     This 
is  a  solution  of  sulphate  of  ammonia.     Here  the  sulphuric  acid  combines 
with  the  ammonia,  and  forms  the  sulphate  of  ammonia,  the  name  of  the 
compound  being  formed  so  as  to  indicate  its  composition.     In  like  man- 
ner, carbonic  acid  united  to  lime  forms  the  compound  of  carbonate  of 
lime  ;  *and  so  on  to  other  cases. 

In  the  same  manner  various  other  salts  may  be  formed. 

4.  Take  a  small  bit  of  phosphorus ;  set  fire  to  it  upon  a  piece  of  glass 

*  This  is  simply  prepared  by  boiling  common  red  cabbage,  cut  into  small 
pieces,  for  a  short  time,  in  no  more  water  than  is  just  sufficient  to  cover 
them. 

f  Litmus  is  a' vegetable  blue. 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

or  tin  placed  in  the  centre  of  a  common  plate,  and  immediately  cover  it 
with  a  large  dry  glass.     The  phosphorus,  as  it  burns,  combines  with  the 
oxygen  of  the  air,  and  thus  forms  phosphoric  acid,  which  rises  in  white 
flakes  within  the  glass,  and  finally  falls  upon 
the  plate  like  snow.     These  flakes  have  a  fine 
acid  taste.  After  the  ignition  has  ceased,  pour 
a  little  water  on  the  plate :  this  dissolves  the 
flakes,  and  a  solution  of  the  acid  is  obtained. 

This  acid,  combining  with  ammonia,  forms 
phosphate  of  ammonia;  with  soda,  it  forms 
phosphate  of  soda ;  with  lime  it  forms  phos- 
phate of  lime,  (which  is  principally  the  com- 
position of  bones  ;)  and  so  on. 

5.  Burn  sulphur  after  the  manner  described 
in  the  last  experiment.  Here  the 'sulphur,  as  it  burns,  combines  with  the 
oxygen  in  the  air,  and  forms  sulphurous  acid,  -v^iich  rises,  in  the  form 
of  a  colorless  gas,  into  the  interior  of  the  glass.  Put  a  violet  flower  (or  a 
piece  of  litmus  paper)  into  the  glass  :  the  color  is  discharged.  A  little 
water  poured  into  the  plate  dissolves  the  gas. 

By  a  peculiar  modification  of  this  process  sulphuric  acid  is  made, 
which  is  a  more  powerful  acid  than  sulphurous  acid,  in  consequence  of 
containing  more  oxygen. 

SOLUTIONS.       . 

12.  When  a  substance  dissolves  in  water,  the  substance  is 
said  to  be  soluble,  and  we  obtain  a  solution  of  it.  The  solu- 
tion of  bodies  in  liquids  presents  us  with  the  most  simple  case 
of  chemical  attraction.  Water  readily  combines  with  sugar, 
common  salt,  sulphuric  acid,  alcohol,  &c. ;  and,  on  the  contrary, 
it  shows  no  tendency  to  unite  with  oil,  ether,  &c.  Camphor 
readily  dissolves  in  alcohol,  but  it  is  almost  insoluble  in  water. 
The  process  of  solution  is  much  accelerated  by  heat  and  agi- 
tation. In  order  to  obtain  a  concentrated  solution  of  some 
substances,  the  liquid  must  be  boiled  in  a  common  oil  flask  for 
some  time  with  the  substance.  Lime  is  sparingly  soluble  in 
water;  yet,  if  a  little  lime  be  added  to  distilled  water,  a 
sufficient  portion  will  be  dissolved  to  indicate  the  presence  of 
lime.  Distilled  or  pure  water  should  be  used  for  making 
solutions ;  however,  in  most  cases,  clean  rain  water  will  do 
very  well. 


EXPERIMENTAL    CHEMISTIU".  455 

EXPERIMENTS. 

1.  Add  a  small  piece  of  camphor  to  alcohol  or  spirits  of  wine ;  stir 
the  mixture ;  the  camphor  is  soon  dissolved,  and  a  clear  solution  of  cam- 
phor in  alcohol  is  obtained. 

Pour  a  little  of  this  solution  into  a  glass  of  water ;  the  alcohol  unites 
with  the  water,  and  leaves  the  camphor  floating  upon  the  surface. 

2.  Add  a  little  lime  to  a  bottle  of  rain  or  distilled  water ;  shake  it 
up ;  and,  after  corking  the  bottle,  set  it  aside  until  the  particles  of  lime 
have  settled  to  the  bottom  :  pour  some  of  the  liquid  into  a  glass,  and  a 
clear  solution  of  lime  is  obtained. 

3.  Dissolve  a  little  carbonate  of  potassa  (pearlash)  in  a  glass  of  water; 
a  clear  solution  of  the  salt  is  thus  obtained ;  add  a  few  drops  of  this 
solution  to  lime  water :  it  becomes  milky,  owing  to  the  formation  of  car- 
bonate  of  lime.     Here  the  carbonic  acid,  having  a  greater  affinity  for 
lime  than  it  has  for  potassa,  combines  with  the  lime,  and  leaves  the 
potassa  in  solution.     The  carbonate  of  lime  is  said  to  be  precipitated  ; 
that  is,  it  falls  to  the  bottom  of  the  glass,  owing  to  its  being  nearly  in- 
soluble. 

4.  Breathe  through  a  tube  into  a  solution  of  lime  :  a  milkiness  is  pro- 
duced, owing  to  the  formation  of  carbonate  of  lime.     Here  carbonic  acid 
gas  is  expired  from  the  lungs. 


SECTION  n. 

FAMILIAR  EXPERIMENTAL  ILLUSTRATIONS  OP  THE  PROP- 
ERTIES AND  COMPOUNDS  OF  SOME  OF  THE  MOST  IM- 
PORTANT SIMPLE  SUBSTANCES. 

CARBON.      CARBONIC    ACID    GAS. 

13.  When  wood  is  burned  (as  is  done  by  the  charcoal 
burners)  in  such  a  manner  as  to  exclude  the  air,  it  is  convert- 
ed into  wood  'charcoal,  which  is  nearly  pure  carbon.  The 
diamond  is  perfectly  pure  carbon  in  a  crystallized  form.  Com- 
bined with  other  substances,  carbon  is  found  in  vegetable, 
animal,  and  many  mineral  substances.  When  charcoal  is 
burned,  in  the  air  it  forms  carbonic  acid  —  a  heavy  gas,  which 
extinguishes  flame,  and  is  destructive  to  animal  life. 


456          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

EXPERIMENTS. 

1.  Put  some  pieces  of  chalk  (carbonate  of  lime)  into  a 
bottle  with  a  wide  mouth ;  add  -sulphuric  acid  (or  any 
other  strong  acid :)    violent  effervescence  takes  place, 
owing  to  the  escape  of  carbonic  acid  gas  with  the  forma- 
tion of  sulphate  of  lime.     Here  the  sulphuric  acid  unites 
with  the  lime  in.  the  chalk,  and  the  carbonic  acid  in  it  is 
set  free  in  the  form  of  gas. 

2.  In  the  last  experiment  the  carbonic  acid  gas,  as  it 
is  formed,  gradually  drives  out  the  air  in  the  bottle,  and 
takes  its  place.     This  gas,  being  colorless,  cannot  be  dis- 
tinguished from  common  air  by  the  eye :  its  presence, 
however,  may  be  detected.      Plunge  a  burning  candle 
into  the  gas :  the  flame  is  instantly  extinguished,  while 

the  gas  remains  unchanged.  pigt  3, 

Thus  carbonic  acid  gas  extinguishes  flame ',  and  at  the  same 
time  it  does  not  takejire,  as  some  other  gases  do. 

HYDROGEN.       COMPOSITION    OF    WATER. 

14.  Hydrogen  is  a  colorless,  inflammable  gas,  and  the 
lightest  known  substance  in  nature,  it  being  about  14£  times 
lighter  than  air.  Water  is  composed  of  hydrogen  and  oxy- 
gen. Hydrogen  also  enters  into  the  composition  of  the  inflam- 
mable or  organic  part  of  plants. 

EXPERIMENTS. 

1.  Put  a  few  pieces  of  zinc  cuttings  into  the  wide-mouthed  bottle,  (see 
last  fig. ;)  pour  upon  them  some  diluted  sulphuric  acid  ;  *  the  mixture 
soon  effervesces,  owring  to  the  escape  of  bubbles  of  hydrogen  gas,  which 
gradually  displace  the  air  and  fill  the  bottle.  Cover  the  bottle  with  a 
plate,  or  with  a  piece  of  window  glass,  t  to  prevent  the  external  air  from 
mingling  with  the  hydrogen.  When  a  sufficient  quantity  has  been 
obtained,  take  off  the  cover,  and  plunge  a  lighted  candle  into  the  gas  : 
the  flame  of  the  candle  is  extinguished,  but  the  gas  takes  fire,  and  burns 
at  the  mouth  of  the  bottle,  with  a  pale  yellow  flame. 

*  A  mixture  of  1  part  of  strong  acid  to  about  4  or  5  parts-  of  water. 

f  N.  B.  In  all  experiments  relative  to  gases  generated  in  this  way,  it 
must  always  be  understood  that  a  plate,  or  a  piece  of  window  glass,  is  to  he 
laid  over  the  mouth  of  the  vessel  for  a  few  seconds,  in  order  to  exclude  the 
external  air. 


EXPERIMENTAL    CHEMISTRY. 


457 


When  hydrogen  is  mixed  with  common  air,  the  ignition  goes  on  more 
rapidly,  and  sometimes  with  a  slight  explosion  ;  but  the  experiment  may 
be  made  with  perfect  safety  in  the  manner  just  described. 

In  this  experiment,  the  sulphuric  acid,  the  oxygen  portion  of  the 
water,  and  the  zinc,  combine  and  form  the  sulphate  of  the  oxide  of 
zinc  ;  which  remains  in  solution,  while  the  hydrogen  portion  of  the 
water  escapes  in  the  form  of  a  gas. 


Thus  hydrogen  burns,  but  does  not  support  flame. 


2.  Generate  hydrogen  in  a  bottle,  as  in  the  last 
experiment ;  and,  after  the  air  has  been  driven  out, 
close  the  mouth  with  a  cork,  through  which  the  tube 
of  a  tobacco  pipe  passes ;  light  the  gas  as  it  issues  from 
the  fine  opening  of  the  tube.  Insert  this  small  flame  a 
few  inches  into  a  glass  tube,  about  twenty  inches  long 
and  one  inch  in  diameter.  As  the  hydjogen  burns,  it 
combines  with  the  oxygen  of  the  air ;  thus  water  is 
formpd,  which  covers  the  interior  of  the  tube  in  the 
form  of  moisture.  After  a  short  time,  the  tube  emits 
musical  sounds.  These  sounds  are  produced  by  the  air 
rushing  in  to  fill  up  the  void  formed  by  the  ignition 
of  the  hydrogen.  To  show  the  formation  of  water,  a 
dry  glass  may  be  held  over  the  flame. 


Fig.  4. 


OXYGEN   AND   NITROGEN.      THE   ATMOSPHERE. 

15.  The  atmosphere  is  a  mixture  of  oxygen  and  nitrogen : 
there  is  also  a  small  portion  of  carbonic  acid  gas  always 
present  in  the  air. 

EXPERIMENTS. 

1.  Put  a  lighted  wax  candle  on  the  table ;  place  over 
it  a  glass  jar,  previously  dried  with  care ;  the  candle 
soon  begins  to  burn  dimly,  as  the  inflammable  substances 
in  it  consume  the  oxygen  of  the  air,  and,  after  a  little 
time,  the  flame  is  extinguished ;  the  interior  of  the  glass 
will  now  be  found  covered  with  drops  of  water.     Here 
the  candle  is  extinguished,  in  consequence  of  the  con- 
sumption of  the  oxygen,  which,  uniting  with  the  hydro- 
gen and  carbon  of  the  tallow,  forms  water  ^nd  carbonic 
acid  gas.     (See  also  Exp.  4,  Art.  11.) 

2.  Put  a  lighted  candle  (supported  by  a  bent  wire  passing  through  a 

39 


Fig.  5. 


458 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  6. 


Fig.  7. 


cork)  into  a  large  bottle ;  close  the  mouth  of  the  bot- 
tle :  the  flame  soon  becomes  dim,  and  then  goes  out,  in 
consequence  of  the  air  being  no  longer  able  to  support 
combustion.  Take  out  the  candle,  rekindle  it,  and 
plunge  it  into  the  bottle  :  the  flame  is  immediately  extin- 
guished. 

If  a  living  animal  were  confined  in  a  close  bottle  after 
the  oxygen  in  the  air  becomes  vitiated,  the  animal  would 
die.  A  second  animal,  placed  in  this  vitiated  air,  would 
at  once  expire. 

3.  Take  a  large  bottle  containing  com- 
mon air  ;  place  its  mouth  in  water ;  close 
the  nostril  with  the  forefinger  and  thumb, 
and  inspire  and  expire  the  air  in  the  bot- 
tle, by  means  of  a  bent  pewter  tube,  for 
a  few  seconds.     At  each  inspiration,  the 
water  rises  in  the  bottle,  and  at  each  ex- 
piration the  water  falls.     Take  the  bottle 
containing  the  air  which  has  thus  been 

vitiated  by  passing  through  the  lungs,  plunge  a  lighted  taper  into  it,  as 
in  Exp.  2  :  the  flame  is  extinguished.  Here  the  oxygen  of  the  air  is 
consumed  in  the  act  of  respiration,  and  the  vitiated  air  returned  to  the 
bottle  contains  the  nitrogen,  which  was  at  first  in  the  air,  mixed  with 
carbonic  acid  gas.  (See  also  Exp.  4,  Art.  12.) 

In  the  process  of  breathing,  the  oxygen  taken  from  the  air 
is  returned  to  it  in  the  form  of  carbonic  acid  gas  ;  thus  one 
great  end  of  breathing  consists  in  depriving  the  blood  of  its 
carbon  or  charcoal. 

Thus  oxygen  not  only  supports  Jtame,  but  also  animal  life : 
hence  it  is  called  vital  air. 

4.  Place  a  wire,  supporting  a  small  cup,  on  a  stand 
or  shelf  covered  with  water ;  put  a  small  piece  of 
phosphorus  in  the  cup ;  ignite  the  phosphorus,  and 
then  invert  a  large  bottle  over  it.     The  phosphorus 
consumes  all  the  oxygen  in  the  bottle,  thereby  form- 
ing phosphoric  acid,  and  leaves  the  nitrogen.     After 
shaking  the  water  in  the  bottle,  (its  mouth  being  still 
kept  under  the  water,)  the  water  rises,  occupying  the 
place  of  the  oxygen  which  hSs  been  consumed.    This 
will  be  found  to  be  about  -5-  of  the  air  at  first  in  the 


Fig.  8. 


bottle.     The  residue  is  nitrogen  gas  ;  thus  showing  that  -J  of  the  bulk 
of  the  air  is  oxygen,  and  f-  are  nitrogen. 


EXPERIMENTAL    CHEMISTRY. 


459 


5.  Take  the  bottle  of  nitrogen  (covering  its  mouth  with  a  piece  of 
glass)  and  place  it  on  the  table  with  its  mouth  uppermost ;  plunge  a 
lighted  candle  into  the  gas  :  the  flame  is  extinguished,  at  the  same  time 
the  gas  does  not  take  fire. 

Thus  nitrogen  neither  supports  flame,  nor  does  it  take  fire  as 
hydrogen  does. 

6.  Put  some  green  leaves  beneath  an  invert- 
ed glass  filled  with  water,  and  place  it  in  the 
sunshine  :  the  leaves  -will  be  found  to  give  off 
oxygen  gas. 

Thus  plants  give  off  oxygen  gas,  while 
animals  consume  it.  Fig.  9. 

7.  Introduce  some   chlorate   of  potassa  in 
powder  (a  salt  which  contains  a  large  quantity 
of  oxygen)  into  a  test  tube ;  apply  the  flame 
of  a  spirit  lamp  :  the  salt  is  decomposed  by  the 
heat,  all  the  oxygen  gas  being  given  off ;  apply 
the  finger  lightly  to  the  mouth  of  the  tube,  to 
keep  the  gas  as  pure  as  possible ;  plunge  a 
lighted  splinter  of  wood  into  the  gas ;    the 
flame  is  much  increased  in  brightness  ;  before 
introduction,  blow  the  flame  out  so  as  to  have 
a  red  spark  remaining :  the  wood  is  instantly 

rekindled,  thereby  showing  that  pure  oxygen  is  an  eminent  supporter 
of  combustion. 

8.  Pour  some  lime  water  into  a  glass,  and  allow  it  to  stand  for  a  few 
hours  :  a  skin  of  carbonate  of  lime  is  formed  upon  the  surface.     This 
shows  that  there  is  carbonic  acid  gas  in  the  atmosphere. 

AMMONIA. 

16.  This  gaseous  substance  is  composed  of  nitrogen  and 
hydrogen.  Water  dissolves  a  large  quantity  of  this  gas,  and 
the  solution  is  called  liquid  ammonia,  or  hartshorn.  It  readily 
combines  with  all  the  acids,  and  forms  salts  of  ammonia. 
This  substance  is  invariably  given  off  from  animal  matter  in 
a  state  of  putrefaction ;  the  ammonia  thus  formed  rises  into 
the  air,  where  it  floats  until  it  is  washed  down  by  the  rains  to 
fertilize  the  soil.  It  is  one  of  the  most  fertilizing  substances 
found  in  farm  yard  manure  and  guano. 


Fig.  10. 


460          NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

EXPERIMENTS. 

1.  Hold  test  papier  over  a  bottle  of  liquid  ammonia  :   a  powerful 
alkaline   action  is  exhibited.     Smell  the  ammonia ;    it  has  a  strong 
pungent  odor. 

2.  Dip  a  glass  rod  in  hydrochloric  acid,  and  hold  it  over  a  bottle 
of  liquid  ammonia :   white  fumes  of  hydrochlorate  of  ammonia  are 
formed. 

3.  Take  a  bottle  of  hydrochloric  acid  into  a  horse  stable ;  take  out 
the  stopple  of  the  bottle :  white  fumes,  as  in  the  last  experiment,  are 
formed  about  the  mouth  of  the  bottle. 

4.  Take  two  bottles ;  put  a  little  liquid  ammonia  into  one 
of  them,  turning  the  bottle  round  so  as  to  spread  the  ammonia 
over  the  interior  ;  in  like  manner  introduce  hydrochloric  acid 
into  the  other  bottle  ;  bring  the  mouths  of  the  bottles  together, 
as  in  the  annexed  cut :  the  dense  white  fumes  of  hydrochlorate 
of  ammonia  are  produced. 

5.  Take  equal  parts  of  hydrochlorate  of  ammonia  (sal  am- 
moniac) and  quick  lime,  each  separately  powdered,  and  mix 
them  briskly  together  ;  the  strong  pungent  fumes  of  ammoni- 

acal  gas  will  be  felt.  Fig'  ll' 

6.  Perform  the  same  experiment  with  a  mixture  of  guano  and  quick 
lime  :  ammonia  is  in  this  case  given  off  from  the  guano. 

7.  To  a  solution  of  carbonate  of  ammonia  add  a  solution  of  oxalic 
acid  until  effervescence  ceases :  a  solution  of  oxalate  of  ammonia  is 
obtained.    Here  the  acid  and  ammonia  combine  with  the  escape  of 
carbonic  acid  gas. 

NITRIC    ACID,   OR  AQUA  FORTIS. 

17.  This  important  substance  is  a  compound  of  nitrogen 
and   oxygen.      It   is   manufactured  from   nitre,  (nitrate   of 
potassa,)  a  substance  composed  of  nitric  acid  and  potassa. 
There  is  reason  to  believe  that  nitric  acid  is  formed  in  the 
air  during  thunder   storms.      Decaying   organic   substances 
containing  nitrogen  yield  this  acid.     Nitric  acid,  as  well  as 
ammonia,  supply  the  growing  plant  with  nitrogen. 

THE   ATMOSPHERE. 

18.  The  atmosphere  is  that  vast  ocean  of  elastic  fluid  which 
every  where  surrounds  the  globe,  extending  to  the  height  of 


EXPERIMENTAL    CHEMISTRY.  461 

about  fifty  miles  above  the  tops  of  our  highest  mountains. 
This  subtle,  elastic  fluid  bears  on  its  tide  the  exhalations  of 
the  earth  over  every  clime,  descends  to  the  lowest  depths 
of  our  mines,  and  penetrates  into  the  recesses  of  our  darkest 
caverns.  Although  invisible  to  the  eye,  and  although  bodies 
move  through  it  with  apparent  ease,  yet  the  chemist  has 
weighed  it  in  his  balance,  and  determined  its  composition  with 
an  exactness  which  challenges  dispute.  Every  where  the 
composition  of  th^  air  is  the  same,*  —  as  far  as  regards  its 
essential  elements,  —  whether  it  be  taken  from  the  confined 
alleys  of  our  crowded  cities,  or  from  the  mountain  tops  over 
which  the  healthful  winds  play  with  unobstructed  freedom. 
Winds,  air  in  motion,  drive  our  vessels  through  the  ocean, 
and  perform  useful  labor  in  our  windmills.  The  atmosphere 
is  the  great  agent  by  which  heat  is  nearly  equally  distributed 
over  the  earth,  and  without  its  agency  light  itself  would 
scarcely  serve  the  purposes  for  which  it  is  designed.  By  its 
means  moisture  is  scattered  over  the  vegetable  creation  in  the 
form  of  rain  and  dew  ;  and  these  rains  wash  down  ammonia, 
nitric  acid,  and  various  exhalations  essential  to  the  growth  of 
plants. 

The  substances  essential  to  the  constitution  of  the  atmos- 
phere are  oxygen,  nitrogen,  carbonic  acid  gas,  and  watery 
vapor.  The  oxygen,  as  we  have  shown,  is  necessary  to  the 
existence  of  the  animal  world,  and  to  the  support  of  combus- 
tion ;  while  the  nitrogen  tends  to  moderate  the  intensity  of 
the  action  of  the  oxygen.  The  comparatively  small  portion 
of  carbonic  acid  gas  in  the  air  affords  an  important  part  of 
the  food  to  the  vegetable  world,  and  the  watery  vapor,  besides 
serving  other  important  purposes,  tends  to  keep  the  skin  of 
animals  and  the  surface  of  plants  in  a  moist  condition.  The 
beautiful  adjustment  of  the  relative  proportion  of  these  sub- 
stances to  suit  the  wants  of  animals  and  plants,  is  a  remarka- 


*  This  arises  from  the  diffusiveness  of  gases,  or  the  tendency  which  they 
have  to  intermix  with  each  other,  without  regard  to  their  difference  of 
density  or  heaviness. 

39* 


462  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

ble  instance  of  the  nice  adaptation  of  means  for  the  produc- 
tion of  a  proposed  end. 

The  air  is  being  continually  supplied  with  carbonic  acid  gas 
from  the  respiration  of  animals ;  from  the  burning  of  wood, 
coal,  and  other  combustible  bodies ;  and  from  all  animal  and 
vegetable  substances  in  a  state  of  decay.  Farm  yard  manure, 
also,  put  into  the  soil  in  a  fermenting  state,  yields  an  abundant 
supply  of  carbonic  acid  gas,  as  well  as  of  ammonia,  to  the 
growing  plant.  The  atmosphere,  howeve*,  affords  the  chief 
source  of  carbonic  acid  to  plants,  which,  assimilating  the  car- 
bon, give  off  the  oxygen  into  the  air,  to  make  up  the  deficien- 
cy produced  by  the  respiration  of  animals.  Guided  by  an 
unseen  power,  one  part  of  creation  administers  to  the  neces- 
sities of  another  part :  thus  plants  and  animals  are  necessary 
to  each  other's  existence,  —  the  one  supplies  what  the  other 
consumes,  —  what  is  discharged  as  useless  from  the  one 
becomes  essential  food  to  the  other.  This  remarkable  law  of 
compensation  seems  to  run  through  the  whole  of  the  universe, 
and  a  proper  appreciation  of  its  nature  cannot  fail  in  forcibly 
impressing  upon  our  minds  the  great  and  solemn  fact  —  that 
the  universe  is  the  work  of  a  Being  infinite  in  wisdom,  good- 
ness, and  truth. 

Thus  the  atmosphere,  which  appears  as  nothing  to  the 
vulgar  eye,  is  not  less  essential  to  the  economy  of  nature,  than 
the  solid  matter  of  which  the  globe  is  composed,  or  the  great 
ocean  of  waters  which  float  upon  its  surface. 

EXPERIMENTS,    WITH    DESCRIPTIONS    OF   PNEUMATIC   APPARATUS. 

1.  Draw  water  into  the  mouth  by  a  tube.  Here  the 
process  of  sucking  draws  the  air  from  the  tube,  and  the 
pressure  of  the  external  air  causes  the  water  to  rise  in  the 
tube.  The  pipette,  used  in  many  chemical  experiments, 
depends  on  this  principle.  When  the  finger  is  placed  upon 
the  upper  opening,  C,  the  fluid  in  the  tube  remains  sus- 
pended ;  and,  on  the  contrary,  when  the  finger  is  removed, 
the  fluid  descends  drop  by  drop  from  the  small  orifice  O  of 
the  lower  extremity.  (For  a  complete  account  of  the 
various  mechanical  properties  of  the  atmosphere,  see  the 
Treatise  on  Pneumatics.) 


EXPEKIM KNTA L    CH KMISTRY. 


403 


2.  Invert  a  bottle  F,  filled  with  water,  in  the  same  fluid  ;  the  water 
rem  ains  suspended  in  the  bottle  by  the  pressure  of  the  external  air.  Blow 
through  a  tube  g  s  t  into  the  mouth  of  the  bottle ;  the  air  rises  in  bubbles 
through  the  water  and  displaces  it. 

This  explains  the  principle  upon  which  the  pneumatic  trough  depends. 
This  simple  piece  of  chemical  apparatus  is  used  for  receiving  different 


Fig.  13. 

kinds  of  gasses  in  bottles  and  gas  receivers  :  it  consists  of  a  rectangular 
trough  W  W,  with  the  shelf  b  b,  having  a  funnel-shaped  hole  passing; 
through  it,  for  placing  the  bottles  and  receivers  on  ;  when  it  is  about  to 
be  used,  water  is  poured  into  the  trough,  so  as  to  cover  the  shelf  to  the 
depth  of  about  an  inch  ;  the  mouth  of  the  bottle  intended  to  receive  the 
gas  is  pkced  over  the  hole  in  the  shelf,  and  the  beak  of  the  retort,  in 
which  the  gas  is  being  formed,  is  placed  immediately  below  this  orifice  : 
the  gas  then  rises  in  the  bottle  and  displaces  the  water. 

In  the  annexed  cut,  r  is  the  retort,  containing  the  mixture  from  which 


Fig.  14. 

the  gas  is  to  be  made,  with  its  beak  placed  below  the  hole  in  the  shelf  w  ; 
T  the  pneumatic  trough  filled  with  water ;  e  the  gas  receiver ;  S  the 


464 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


15. 


retort  stand,  with  its  ring  supporting  the  retort  r ;  A  an  Argand  lamp,  with 
its  chimney  c,  for  applying  a  steady  heat  to  the  retort  when  it  is  required 
for  generating  the  gas  :  some  gases,  however,  such  as  hydrogen,  for  ex- 
ample, are  given  off  from  the  materials  in  the  retort  without  the  aid  of 
external  heat. 

N.  B.  In  the  preparation  of  gases  in  this  way,  it  should  be  observed 
that  the  gas  which  first  comes  over  is  mixed  with  the  atmospheric  air  in 
the  retort ;  hence  a  volume  of  gas  equal  to  about  twice  the  volume  of 
the  retort  should  be  thrown  away  as  impure ;  this  should  especially  be 
attended  to  in  the  case  of  gases  (such  as  hydrogen)  which  detonate  when 
mixed  with  atmospheric  air. 

Gases  may  be  transferred  from  one  vessel  to 
another,  over  the  pneumatic  trough.  In  order  to 
transfer  the  gas  from  e  to  b,  bring  the  lower  edge 
of  e  to  the  mouth  of  b  ;  gradually  depress  the  up- 
per end  of  e  ;  bubbles  of  gas  will  pass  from  e  and 
fill  the  vessel  b. 

When  a  large  quantity  of  gas  is  to  be  made,  the 
gas  holder  is  preferable  to  the  pneumatic  trough. 
This  valuable  piece  of  apparatus  consists  of  a  closed 

cylindrical  vessel  A,  and  a  shelf  B,  open  at  the  top,  supported  on  three 
rods  ;  a  c  is  a  pipe,  open  at  each  extremity,  reaching  from  the  bottom  of 
the  shelf  to  the  bottom  of  the  cylinder  ;  e  b 
is  another  pipe  which  merely  enters  the  top 
of  the  cylinder ;  communications  can  be 
opened  by  the  cocks  a  b  ;  d  is  a  cock  through 
which  the  gas  in  the  cylinder  may  be  drawn 
off;  h  is  an  aperture  for  introducing  the 
pipe  which  conducts  the  gas  into  the  re- 
ceiver A  ;  f  g  is  a  glass  tube  opening  into 
the  cylinder  at  the  top  and  bottom,  to  show 
the  quantity  of  gas  that  may  be  in  the  cyl- 
inder at  any  time.  To  fill  the  cylinder  A 
with  gas  :  A  is  first  filled  with  water,  which 
is  done  by  opening  the  three  cocks  a,  b,  d, 
closing  the  aperture  h  with  a  cork,  and 
pouring  water  into  the  shelf  B  :  the  water 
runs  through  the  pipes  a  and  b  into  A,  ex- 
pelling the  air  through  d.  When  A  is  filled 
with  water,  the  cocks  a,  6,  and  d  are  closed, 
and  the  aperture  h  is  opened  ;  the  water  re- 
mains in  the  cylinder  A  in  consequence  of 
the  atmospheric  pressure,  just  in  the  same  way  as  water  is  suspended  in 


1C. 


EXPERIMENTAL    CHEMISTRY.  465 

the  bird  fountain  ;  introduce  the  tube  proceeding  from  the  retort  into  the 
aperture  h ;  the  gas  will  then  rise  in  bubbles  into  the  cylinder,  displacing 
the  water  through  h.  To  fill  a  jar  B  with  the  gas  :  Pour  water  on  the 
shelf  B  ;  open  the  cocks  b  and  a  ;  the  gas  then  rises  in  bubbles  into  B, 
from  the  pressure  produced  by  the  column  of  water  in  the  pipe  a  c.  The 
gas  may  also  be  transferred  through  the  cock  d. 


SULPHUR. 

19.  This   important  elementary  substance  abounds,  in  its 
simple   state,  in  the  Island  of  Sicily,  and  in  many  volcanic 
countries.     It  is  also  found  in  combination  with  iron,  and  cop- 
per, in  many  parts  of  the  world.     Sulphuric  acid  is  the  most 
important  compound  of  sulphur ;  united  with  various  bases, 
such  as  lime,  soda,  magnesia,  &c.,  it  forms  sulphates,  which 
are  found  abundantly  in  the  mineral  kingdom. 

EXPERIMENTS. 

1.  Heat  sulphur  in  a  test  tube;  the  sulphur  first  melts,  and  then 
rises  in  vapor,  which  condenses  in  the  cold  part  of  the  tube.     (See  also 
Exp.  1,  Art.  1,  and  Exp.  5,  Art.  11.) 

2.  To  a  solution  of  baryta  add  sulphuric  acid  ;  the  white  precipitate 
of  sulphate  of  baryta  falls,  which  is  not  dissolved  by  nitric  acid.     This 
is  the  best  test  for  the  presence  of  sulphuric  acid. 

3.  Put  some  sulphuret  of  iron  in  a  bottle,  and  pour  some  diluted  sul- 
phuric acid  upon  it ;  sulphuretted  hydrogen  gas  is  given  off,  which  has 
the  smell  of  rotten  eggs ;  dip  a  slip  of  white  paper  in  a  solution  of 
acetate  of  lead,  and  suspend  the  paper  in  the  bottle  containing  the  gas  : 
the  paper  is  rendered  black  from  the  formation  of  the  sulphuret  of  lead. 

Sulphuretted  hydrogen^  or  hydrosulphuric  acid,  is  highly  inflammable, 
and  is  much  used  as  a  test  for  the  presence  of  different  kinds  of  metals. 
The  fumes  of  this  gas  should  be  avoided,  as  it  is  deleterious  to  animal 
life.* 

PHOSPHORUS. 

20.  This  elementary  substance  is  very  inflammable,  and 
therefore  should  be  handled  with  great  caution.     It  has  very 

*  All  fumes  given  off  by  chemical  action  should  be  carefully  avoided. 


4G6  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

much  the  appearance  and  consistence  of  wax.     Phosphorus 
is  now  universally  prepared  from  bones. 

EXPERIMENTS. 

1.  Fold  a  thin  slice  of  dried  phosphorus  in  a  piece  of  paper ;  rub  it 
briskly  with  any  smooth  body :  the  heat  produced  by  the  friction  speed- 
ily ignites  the  phosphorus. 

2.  Write  upon  the  wall  with  a  stick  of  phosphorus,  (wrapped  round 
with  a  piece  of  paper  ;)  the  writing  appears  luminous  in  the  dark.   (See 
alsoExp.  4,  Art.  11.) 

3.  Phosphuretted  hydrogen  gas.  —  Put  some  zinc  cuttings  and  a  few 
small  slices^of  phosphorus  into  a  glass  tumbler ;  take  the  glass  into  a 
dark  room,  and  add  some  diluted  sulphuric  acid  :  the  mixture  appears 
like  a  well  of  fire,  in  consequence  of  the  escape  of  phosphuretted  hydro- 
gen gas,  which  ignites  spontaneously  when  it  comes  into  the  air. 


IODINE. 

21.  This  elementary  substance  is  solid,  having  a  dark  bluish 
color,  with  a  somewhat  metallic  lustre.  It  is  found  in  sea 
water  and  marine  plants.  It  is  highly  soluble  in  alcohol,  but 
is  sparingly  dissolved  by  water.  Its  most  important  compound 
is  iodide  of  potassium,  which  is  now  much  used  as  a  medicine. 

EXPERIMENTS. 

1.  Heat  one  or  two  grains  of  iodine  in  a  flask :  the  beautiful  violet 
vapor  of  iodine  rises  within  the  flask,  and  slowly  condenses. 

2.  Dissolve  a  nenj  small  piece  of  iodine  in  water  :  the  water  has  a 
brown  color.     To  this  solution  add  a  cold  solution  of  starch  :  *  the  beau- 
tiful blue  compound  of  iodide  of  starch  is  formed. 

3.  Drop  a  small  piece  of  iodine  on  a  few  grains  of  phosphorus  :  the 
substances  combine  with  ignition. 

4.  To  a  cold  solution  of  starch  add  a  few  drops  of  iodide  of  potassium  ; 
110  action  is  produced  :  add  now  a  little  sulphuric  acid  to  set  the  iodine 
free  ;  the  blue  iodide  of  starch  is  formed. 

*  Starch  should  be  dissolved  in  hot  water. 


EXPERIMENTAL    CHEMISTRY.  467 


CHLORINE. 

22.  Chlorine  is  a  greenish-yellow  gas,  (hence  its  name,) 
which  has  a  pungent,  suffocating  odor  ;  it  is  not  inflammable, 
but  it  supports  combustion  ;  indeed,  some  bodies  ignite  in  it 
spontaneously.  It  combines  with  the  metals,  forming  chlorides  ; 
thus  common  salt  is  a  chloride  of  sodium.  With  oxygen  it 
forms  acids  ;  the  most  important  of  these  is  chloric  acid, 
which,  combined  with  potassa,  forms  chlorate  of  potassa,  a 
salt  largely  employed  in  the  manufacture  of  lucifer  matches. 
Chlorine  destroys  all  coloring  matters  and  offensive  effluvia. 

EXPERIMENTS. 

1 .  Put  a  table  spoonful  of  chloride  of  lime  (common  bleaching  powder) 
into  a  bottle  ;  add  an  equal  bulk  of  hydrochloric  acid ;  chlorine,  in  the 
form  of  a  greenish-yellow  gas,  soon  fills  the  bottle  :  introduce  a  lighted 
candle ;  it  burns  with  a  dull  red-colored  name  in  the  gas ;  suspend  a 
moistened  slip  of  blue  litmus  paper  (or  any  other  colored  substance)  in 
the  gas  :  the  paper  is  soon  bleached  by  the  gas. 

2.  To  a  mixture  of  common  salt  and  black  oxide  of  manganese  add 
sulphuric  acid ;  chlorine  gas  is  given  off.     This  is  a  highly  convenient 
way  of  using  chlorine  for  purposes  of  fumigation.    The  chlorine  destroys 
all  noxious  malaria. 

3.  Add  hydrochloric  acid  so  as  to  cover  half  a  tea  spoonful  of  chlorate 
of  potassa  in  powder,  in   a  small  bottle ;  chlorine  gas  (mixed  with 
chlorous  acid)  is  generated  ;  dip  a  slip  of  writing  paper  into  oil  of  tur- 
pentine, and  introduce  it  into  the  gas^  combustion  immediately  takes 
place.     Perform  the  bleaching  experiment  described  in  Exp.  1. 

Chlorous  acid  explodes  with  great  violence,  when  heated 
even  to  a  moderate  temperature. 

4.  Mix  a  few  grains  of  powdered  lump  sugar  with  twice 
the  quantity  of  chlorate  of  potassa ;  let  fall  a  drop  of  sul- 
phuric acid  on  the  mixture:  chlorous  acid  is  disengaged, 
which  immediately  inflames  the  mixture. 

5.  Carefully  fold  in  a  piece  of  paper  a  little  chlorate  of 
potassa  in  powder,  with  a  small  piece  of  phosphorus  ;  strike 
the  mixture  with  a  hammer  :  a  loud  explosion  takes  place. 

6.  To  inflame  phosphorus  under  ivatcr.  —  Put  some  crys- 
tals of  chlorate  of  potassa,  together  with  a  few  slices  of  phos- 
phorus,  into  an  ale  glass  ;  fill  the  glass  with  cold  water ;  let 


468          NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 

fall  a  few  drops  of  sulphuric  acid,  by  means  of  a  pipette,  on  the  chlorate 
of  potassa :  the  acid  takes  up  the  potassa  from  the  salt,  and  sets  free  a 
compound  of  chlorine  and  oxygen,  which  inflames  the  phosphorus. 


HYDROCHLORIC    ACID. 

23.  Hydrochloric  acid,  or  muriatic  acid,  is  a  gas,  composed 
of  hydrogen  and  chlorine  ;  it  is  largely  dissolved  by  water, 
forming  common  aqueous  hydrochloric  acid. 

EXPERIMENT. 

Add  diluted  sulphuric  acid  to  common  salt,  in  a  bottle ;  hydrochloric 
acid  gas  is  given  off  with  effervescence,  and  fills  the  bottle ;  suspend  a 
slip  of  moist  blue  litmus  paper  in  the  gas  :  the  color  is  changed  to  red  ; 
plunge  a  lighted  candle  into  the  gas  :  the  fiame  is  extinguished. 

In  this  experiment  the  acid  decomposes  the  salt,  which  is  a  compound 
of  chlorine,  sodium,  and  water  ;  the  hydrogen  of  the  water  unites  with 
the  chlorine,  and  forms  hydrochloric  acid  gas ;  and  the  oxygen  of  the 
water  unites  with  the  sodium,  and  forms  soda,  which  combines  with  the 
sulphuric  acid,  and  forms  the  sulphate  of  soda. 


SECTION  III. 


METALS    AND    METALLIC    OXIDES. 
POTASSA   AND    SODA. 

24.  Potassium  and  sodium,  united  with  oxygen,  form 
potassa  and  soda.  These  important  substances  are  called 
fixed  alkalies,  to  distinguish  them  from  ammonia,  which  is 
called  the  volatile  alkali.  (See  Art.  11.)  Potassa  is  found 
in  the  ashes  of  plants,  and  soda  in  the  salt  of  sea  water. 

EXPERIMENTS. 

1.  Throw  a  grain  of  potassium  upon  water ;  it  floats  on  the  water,  and 
takes  fire  :  a  solution  of  potassa  is  formed  by  the  union  of  the  oxygen 
of  the  water  with  the  metal. 


EXPERIMENTAL    CHEMISTRY. 


469 


2.  Burn  some  pieces  of  wood  ;  collect  the  ashes,  and  pour  water  upon 
them  to  dissolve  the  potassa  which  is  in  them  ;  add  a  solution  of  some 
vegetable  blue  :  the  color  is  changed  to  green. 

3.  Boil,  in  an  iron  vessel,  equal  weights  of  slaked 
lime  and  carbonate  of  potassa  (pearl  ashes)  in  abput 
twelve  times  the  weight  of  water ;   the  carbonic 
acid  unites  with  the  lime,  forming  the  insoluble 
carbonate  of  lime,  leaving  the  potassa  in  solution. 
Cover  the  mixture,  and  allow  it  to  stand  until  the 
carbonate  of  lime  subsides  ;  draw  the  clear  solution 
off  by  means  of  a  siphon.*     When  a  solution  of 
potassa  is  exposed  to  the  air,  it  speedily  takes  up 
carbonic  acid,  and  returns  to  the  state  of  carbonate 
of  potassa. 

4.  To  a  strong  solution  of  carbonate  of  potassa  add  a  solution  of  tar- 
taric  acid  ;  crystals  of  bitartrate  of  potassa  (cream  of  tartar)  are  formed 
with  the  escape  of  carbonic  acid  gas. 

5.  Boil  nitrate  of  potassa  (nitre)  in  water,  so  long  as  any  of  the  salt 
is  taken  up  ;  decant  the  solution,  and  as  it  cools.,  crystals  of  nitre,  in  six- 
sided  prisms,  are  deposited. 

Soda  is  found  in  the  ashes  of  sea  weed  ;  it  is  also  obtained  from  com- 
mon salt.     The  compounds  of  soda  are  very  similar  to  those  of  potassa. 


Fig.  18. 


LIME. 


25.    Chalk,  limestone,  marble,  lime  shell,  and  calcareous 
spar,  are  all  compounds  of  lime  and  carbonic  acid.     Lime 


*  Insoluble  substances,  or  precipitates,  are 
usually  separated  from  liquids  by  FILTRATION, 
which  consists  in  passing  the  liquid  through  fil- 
tering paper  placed  in  a  funnel  f ;  by  this  pro- 
cess the  clear  liquid  drops  into  the  glass  g,  and 
the  precipitate  or  insoluble  substance  remains  on  g 
the  filtering  paper. 

These  niters  are  formed  by  making  two  folds, 
in  a  round  piece  of  paper,  at  right  angles  to  each 
other,  and  in  a  contrary  direction ;   when  this 
piece  of  paper  is  placed  within  the  funnel,  it  will  assume  the 
form  of  p,  shown  in  the  cut.     The  liquid  to  be  filtered  should 
be  carefully  poured  upon  the  sides  of  the  filter,  so  as  not  to  in- 
jure the  paper  at  the  bottom  part.     Before  use,  the  filter  paper 
should  be  moistened  with  distilled  water.  pig  20 

40 


Fig.  19. 


4:70  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

forms  an  essential  constituent  of  all  good  soils.  Mixed  with 
vegetable  or  animal  substances,  it  promotes  their  decay,  and 
at  the  same  time  absorbs  the  noxious  gases  that  are  given  off. 
Lime  is  an  oxide  of  a  metal  called  calcium. 

EXPERIMENTS 

1.  Expose  lime  water,  in  an  open  vessel,  to  the  air  ;  a  crust  of  car- 
bonate of  lime  soon  appears  upon  the  surface.     See  also  Experiments  2, 
3,  and  4,  Art.  12. 

2.  Pour  hydrochloric  acid  upon  some  pieces  of  chalk,  so  long  as  any 
effervescence  is  seen  :  a  solution  of  hydrochlorate  of  lime  is  formed. 

3.  Make  a  solution  of  nitrate  of  lime,  by  adding  nitric  acid  to  chalk, 
after  the  manner  of  the  last  experiment. 

4.  Pour  a  little  of  the  solution  of  hydrochlorate  of  lime  into  an  ale 
glass,  and  about  the  same  quantity  of  strong  sulphuric  acid  into  another 
glass  ;  pour  the  latter  quickly  upon  the  former  ;  a  violent  effervescence 
takes  place  from  the  escape  of  hydrochloric  acid :  a  .solid  white  sub- 
stance, sulphate  of  lime,  is  formed.     Owing  to  the  condensation,  great 
heat  is  evolved. 

5.  To  any  solution  of  lime  add  'oxalate  of  ammonia ;  (see  Exp.  7, 
Art.  16  :)  the  white  insoluble  oxalate  of  lime  falls. 

MAGNESIA. 

26.  This  substance  is  found  in  sea  water,  in  certain  varieties 
of  limestone,  (magnesian  limestone,)  and  in  many  spring 
waters.  Magnesia  is  the  oxide  of  a  metal  called  magnesium. 

EXPERIMENTS. 

1.  To  diluted  sulphuric  acid  add  carbonate  of  magnesia  (a  white 
powder)  until  effervescence  ceases  :  a  solution  of  sulphate  of  magnesia 
(Epsom  salts)  is  obtained. 

Boil  off  or  evaporate  a  portion  of  the  water ;  *  set  aside  the  solution 
until  it  cools  :  crystals  of  the  salt  will  be  formed. 

2.  To  a  solution  of  sulphate  of  magnesia  add  a  solution  of  carbonate 
of  potassa  :  a  white  precipitate  of  carbonate  of  magnesia  is  formed. 

*  Evaporations  are  best  conducted  in  porcelain  dishes,  or,  as  they  are 
called,  evaporating  dishes  ;  the  heat  should  be  applied  by  a  sand  bath,  or  by 
an  Argand  lamp  with  a  tin  or  copper  chimney. 


EXPERIMENTAL    CHEMISTRY.  471 

This  distinguishes  Epsom  salts  from  oxalic  acid,  a  poison  frequently 
mistaken  for  the  former.  It  is  further  to  be  observed,  that  oxalic  acid  is 
sour  to  the  taste,  whereas  Epsom  salts  are  bitter.  Oxalic  acid  is  dissi- 
pated when  thrown  upon  hot  cinders,  whereas  Epsom  salts  leave  a  white 
mass  behind. 

ALUMINA. 

27.  This  earth  is  an  oxide  of  a  metal  called  aluminum ;  it 
abounds  in  common  clay.     It  is  distinguished  by  its  insolubil- 
ity, and  by  being  dissolved  in  a  solution  of  potassa.     Alum  is 
one  of  its  most  useful  and  common  compounds.     This  salt 
contains  alumina,  potassa,  and  sulphuric  acid.     Pure  clay  is  a 
compound  of  silica  and  alumina,  in  the  proportion  of  about 
3  parts  of  the  former  to  2  of  the  latter. 

EXPERIMENTS. 

1.  Add  ammonia  to  a  solution  of  alum :  alumina  falls  in  consequence 
of  the  ammonia  combining  with  a  portion  of  the  acid. 

2.  Perform  the  same  experiment,  using  potassa  or  soda. 

3.  In  a  saturated  solution  of  alum  suspend  a  basket  formed  of  wool- 
len  thread :  the  alum  forms  beautiful  crystals  on  the  thread,  thereby 
forming  an  alum  basket. 

SILICA. 

28.  This  earth,  like  alumina,  is  very  abundant  in  nature. 
Quartz  is  nearly  pure  silica,  and  it  is  the  chief  ingredient  in 
sand  and  common  flint.     Mixed  with  clay,  it  forms  the  great 
body  of  soils.     Silica  is  an  oxide  of  silicon. 

EXPERIMENTS. 

1.  Mix  one  part  of  fine  sand  with  three  parts  of  carbonate  of  potassa ; 
fuse  the  mixture  in  a  crucible ;  carbonic  acid  is  driven  off,  and  the  sih'ca 
and  potassa  combine  and  form  a  glass,  called  silicated  potassa,  which 
readily  dissolves  in  water ;"  pour  out  the  silicated  potassa  on  an  iron 
plate ;  dissolve  a  portion  of  it  in  water.     This  experiment  is  highly  im- 
portant, considered  in  relation  to  agricultural  science. 

2.  To  the  solution  of  silicated  potassa  add  a  solution  of  hydrochlorate 
of  ammonia ;  the  hydrochloric  acid  combines  with  the  potassa,  and  the 
sih'ca  is  precipitated. 


472          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


IRON. 

29.  This  valuable  metal  is  found  in  a  great  variety  of  forms 
in  nature.  Combined  with  oxygen,  it  is  found  as  an  oxide  of 
iron ;  with  sulphur,  as  a  sulphuret  of  iron ;  with  carbonic  acid, 
as  a  carbonate  of  iron. 

EXPERIMENTS. 

1 .  Place  some  iron  filings  in  a  saucer  ;  moisten  them  from  day  to  day, 
until  they  become  rust,  or  oxide  of  iron,  by  combining  with  oxygen. 

2.  To  some  iron  filings  add  diluted  sulphuric  acid :  hydrogen  gas  is 
given  off,  and  a  solution  of  iron  (green  vitriol)  is  formed.     Here  the" 
oxygen  of  the  water  combines  with  the  iron,  forming  oxide  of  iron, 
which  unites  with  the  acid,  forming  the  sulphate  of  the  oxide  of  iron, 
or,  as  it  is  simply  called,  sulphate  of  iron. 

Decant  the  clear  solution,  evaporate  it,  and  set  it  aside ;  when  the 
solution  is  cold,  green  crystals  will  appear. 

3.  Add  a  few  drops  of  a  strong  solution  of  sulphate  of  iron  to  four 
glasses  containing  water :  — 

i.  To  the  first  add  a  solution  of  potassa :  oxide  of  iron  falls. 
ii.  To  the  second  add  a  solution  of  carbonate  of  potassa  :  carbonate 

of  iron  falls. 

m.  To  the  third  add  a  solution  of  prussiate  of  potassa  :  a  fine  blue 
precipitate  of  Prussian  blue  is  formed. 

In  these  three  experiments,  the  sulphuric  acid  combines  with  the 
potassa,  and  remains  in  solution. 

rv.  To  the  fourth,  add  an  infusion  of  galls :  the  black  gallate  t)f 
iron,  the  substance  which  gives  the  color  to  ink,  after  a  few 
seconds  appears. 

4.  To  a  glass  of  water  add  a  few  drops  of  ink ;  add  oxalic  or  hydro- 
chloric acid  :  the  color  disappears. 

5.  "Write  on  paper  with  a  very  diluted  solution  of  sulphate  of  iron  ; 
when  dry,  the  writing  is  invisible ;  wash  it  over  with  a  solution  of  prus- 
siate of  potassa :  the  writing  appears  of  a  fine  blue  color. 


COPPER. 

30.  This  metal  exists  in  nature  in  its  pure  or  metallic  state ; 
but  it  is  chiefly  found  as  a  sulphuret  of  copper,  (copper 
pyrites.) 


EXPERIMENTAL    CHEMISTRY.  473 

• 

EXPERIMENTS. 

1.  Heat  copper  for  some  time  in  the  fire  ;  suddenly  plunge  the  heated 
copper  into  water :  the  oxide  of  copper  is  formed  in  scales  on  the  surface 
of  the  metal. 

2.  Put  some  slips  of  copper  into"  diluted  nitric  acid,  which  is  colorless : 
a  portion  of  the  copper  is  soon  dissolved  by  the  nitric  acid,  and  a  fine 
blue  solution  of  nitrate  of  copper  is  formed.     Here  a  portion  of  the  acid 
gives  up  oxygen  to  the  metal,  forming  oxide  of  copper,  which  combines 
with  the  nitric  acid.     Red  fumes  of  nitrous  acid  are  given? off. 

By  evaporation,  this  salt  may  be  obtained  in  crystals. 

3.  Into  a  solution  of  sulphate  of  copper  (blue  vitriol)  dip  a  clean  piece 
of  iron :  the  plate  is  covered  with  metallic  copper.     Here  the  copper  is 
precipitated  in  consequence  of  the  iron  uniting  with  the  acid  to  form 
sulphate  of  iron. 

4.  Add  two  drops  of  a  strong  solution  of  sulphate  of  copper  to  two 
glasses  containing  water :  these  solutions  will  be  nearly  colorless. 

i.  To  the  first  add  a  drop  of  ammonia ;  light  blue  oxide  of  copper 
falls :  add  ammonia  now  in  excess ;  the  precipitate  is  re- 
dissolved,  and  the  solution  assumes  a  fine  deep-blue  color. 
This  is  a  very  delicate  .test  of  the  presence  of  copper. 

ii.  To  the  second  add  carbonate  of  potassa :  light  blue  carbonate 
of  copper  falls. 

5.  Place  a  few  crystals  of  nitrate  of  copper  on  a  piece  of  tift  foil ;  add 
a  few  drops  of  water  to  the  crystals,  and  quickly  fold  up  the  tin  foil 
round  them  :  a  violent  chemical  action  takes  place,  and  the  tin  foil  in- 
flames. 

LEAD. 

31.   The  most  common  native  form  of  lead  is  sulphuret  of 
lead,  or  galena. 

EXPERIMENTS. 

1.  Heat  lead  in  an  iron  spoon:  it  soon  melts,  and  then  oxidates,  by 
taking  oxygen  from  the  air. 

2.  Arrange  seven  glasses,  each  containing  a  diluted  solution  of  acetate 
of  lead,  (sugar  of  lead.) 

i.  To  the  first  add  an  alkali :  the  oxide  of  lead  falls, 
ir.  To  the  second  add  carbonate  of  potassa :  the  white  carbonate 

of  lead  (white  lead)  falls. 

in.  To  the  third  add  sulphuric  acid,  or  any  sulphate :  white  suj-? 
phate  of  lead  falls. 
40* 


474         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

rv.  To  the  fourth  add  hydrochloric  acid :   white  chloride  of  lead 

falls. 

v.  To  the  fifth  add  a  few  drops  of  a  solution  of  iodide  of  potas- 
sium :  the  beautiful  yellow  iodide  of  lead  falls, 
vi.  To  the  sixth  add  a  few  drops  of  the  solution  of  chromate  of 

potassa :  yellow  chromate  of  lead  falls. 

vii.  To  the  seventh  add  hydrosulphuret  of  ammonia :  the  black 
sulphuret  of  lead  falls.     (See  Exp.  4,  Art.  60.) 

3.  Suspend  a  piece  of  zinc  in  a  moderately  strong  solution  of  acetate 
of  lead  :  the  lead  appears  deposited  on  the  zinc  in  an  arborescent  form, 
producing  what  is  called  the  lead  tree.  Here  the  zinc  takes  the  place  of 
the  lead,  and  the  latter  is  precipitated. 

CHROME. 

32.  The  most  common  salt  of  this  metal  is  bichromate  of 

potassa. 

EXPERIMENTS. 

Arrange  four  glasses,  each  containing  a  diluted  solution  of  bichromate 
of  potassa. 

1.  To  the  first  add 'carbonate  of  potassa  :  it  unites  with  the  excess  of 
acid,  and  yellow  chromate  of  potassa  appears. 

2.  To  the  second  add  acetate  of  lead.     (See  Exp.  2,  Art.  31.) 

3.  To  ftie  third  add  a  few  drops  of  the  nitrate  of  mercury:  the 
orange- colored  chromate  of  mercury  falls. 

4.  To  the  fourth  add  a  few  drops  of  the  nitrate  of  silver,  (lunar 
caustic  :)  brick-red  chromate  of  silver  falls. 

MERCURY. 

33.  This  metal  is  sometimes  found  native  in  the  metallic 
form,  but  it  is  most  commonly  combined  with  sulphur.     This 
metal  is  a  fluid. 

EXPERIMENTS. 

1.  Heat  a  few  grains  of  mercury  in  a  test  tube  over  the  spirit  lamp  : 
the  mercury  rises  in  vapor,  and  condenses  in  globules  in  the  cold  part  of 
the  tube. 

2.  Heat  a  little  sulphur,  with  about  five  times  its  weight  of  mercury, 
in  a  test  tube :  close  the  mouth  of  the  tube  lightly  $ith  the  forefinger : 
vermilion,  or  bisulphuret  of  mercury,  is  formed. 

3.  To  a  solution  of  chloride  of  mercury  (corrosive  sublimate)  add  a 
few  drops  of  iodide  of  potassium :  a  red  biniodide  of  mercury  falls. 


EXPERIMENTAL    CHEMISTRY.  475 

4.  Heat  some  mercury  with  nitric  acid;  the  mercury  takes  oxygen 
from  a  portion  of  the  acid,  and  combines  with  the  other  portion ;  and  a 
solution  of  nitrate  of  mercury  is  formed. 

ZINC. 

34.  This  metal  is  now  much  used  for  making  water  pipes 
and  spouts. 

EXPERIMENTS. 

1.  Take  the  solution  of  sulphate  of  zinc  obtained  by  Exp.  1,  Art.  14  ; 
evaporate  a  portion  of  the  water  off;  set  the  liquid  aside  to  cool :  pris- 
matic crystals  of  sulphate  of  zinc  fall. 

2.  To  a  solution  of  sulphate  of  zinc  add  a  few  drops  of  ammonia,  (or 
potassa  :)"  white  oxide  of  zinc  falls.     Add  ammonia  hi  excess  :  the  pre- 
cipitate is  completely  redissolved. 

3.  To  a  solution  of  zinc  add  a  few  drops  of  the  carbonate  of  ammo- 
nia: carbonate  of  zinc  falls,  which  is  redissolved  by  an  excess  of  the 
precipitant.     These  two  experiments  form  the  tests  for  the  presence 
of  zinc. 

SILVER. 

35.  Silver  is  distinguished  by  its  brilliant  lustre  and  fine 
white  color. 

EXPERIMENTS. 

1.  To  a  few  small  pieces  of  silver  add  diluted  nitric  acid ;  apply  heat 
until  the  acid  ceases  to  give  off  fumes :  a  solution  of  nitrate  of  silver  is 
obtained ;  as  the  solution  cools,  crystals  are  deposited. 

2.  To  a  solution  of  nitrate  of  silver  add  potassa :  an  ash-gray  powder 
of  oxide  of  silver  falls. 

3.  To  a  very  diluted  solution  of  nitrate  of  silver  add  hydrochloric 
acid :  chloride  of  silver  falls  in  the  form  of  a  white,  curdy  substance, 
which  soon  becomes  black  upon' exposure  to  the  light. 

4.  Write  upon  linen  with  a  solution  of  nitrate  of  silver,  and,  when 
the  writing  is  dry,  wash  it  with  a  solution  of  potassa  :  the  writing  soon 
becomes  permanently  black,  owing  to  the  formation  of  oxide  of  silver. 

GOLD. 

36.  This  metal  is  not  affected  by  exposure  to  the  air,  and 
ordinary  acids  produce  no  action  upon  it. 


476          NATURAL   AND    EXPERIMENTAL   PHILOSOPHY. 

EXPERIMENTS. 

1.  Put  five  or  six  gold  leaves  into  a  test  tube ;  pour  upon  them  a  few 
drops  of  a  mixture  of  nitric  and  hydrochloric  acids ;  apply  the  flame  of 
a  spirit  lamp  :  the  gold  leaves  are  dissolved.     Continue  to  apply  a'gentle 
heat,  so  as  to  expel  any  excess  of  acid  :  terchloride  of  gold  remains.     In 
this  process  chlorine  is  set  free  from  the  hydrochloric  acid,  and  combines 
with  the  gold. 

2.  'Cover  a  slip  of  glass  with  a  few  drops  of  the  terchloride  of  gold  ; 
apply  the  flame  of  a  spirit  lamp :  the  chlorine  is  expelled,  and  gold  is 
left  upon  the  glass. 

3.  Put  a  drop  of  chloride  of  mercury  on  a  gold  ring  ;  with  the  point 
of  a  penknife  touch  the  gold  through  the  drop :  a  permanently  white 
spot  of  an  amalgam  of  gold  is  produced. 

PLATINUM. 

37.  This  metal  is  much  used  for  making  different  kinds  of 
chemical  apparatus,  on  account  of  it  being  very  infusible,  and 
scarcely  at  all  acted  upon  by  ordinary  chemical  agents. 

EXPERIMENTS. 

1.  Mix  nitric  acid  with  an  equal  bulk  of  hydrochloric  acid  ;  add  the 
mixture  to  a  few  small  pieces  of  platinum  wire  in  a  Florence  flask ;  di- 
gest, —  that  is,  keep  the  liquid  at  a  slow  boiling  heat,  —  until  the  acid  is 
neutralized :  a  solution  of  bichloride  of  platinum  is  formed. 

To  obtain  it  perfectly  free  from  acid,  evaporate  cautiously  to  dryness, 
and  dissolve  the  residue  in  water. 

2.  Add  a  drop  of  the  solution  of  bichloride  of  platinum  to  a  glass  of 
water  ;  into  this  solution  let  fall  a  drop  of  iodide  of  potassium :  a  deep 
port  wine  colored  compound  is  immediately  produced.     The  delicacy  of 
this  test  is  truly  remarkable. 

3.  To  the  solution  of  bichloride  of  platinum  add  a  solution  of  hydro- 
chlorate  of  ammonia :   a  yellow  precipitate  is  formed,  a  compound  of 
this  salt  and  platinum. 

Decant  the  liquid,  and  dry  the  precipitate ;  put  it  into  the  .bowl  of  a 
tobacco  pipe,  and  bring  it  to  a  good  red  heat  in  the  fire  :  metallic  plati- 
num, in  a  spongy  state,  is  left,  the  other  substances  having  been  ex- 
pelled by  the  heat. 

4.  Hold  the  spongy  platinum  before  a  stream  of  hydrogen  gas :  the 
metal  soon  becomes  red  hot,  and  the  gas  is  ignited. 


EXPERIMENTAL    Q^Q  477 


SECTION  IV. 

DOCTRINE    OF   EQUIVALENTS.       CHEMICAL     NOMENCLATURE, 
SYMBOLS,    ETC. 

38.  When  bodies  combine  with  each  other,  it  is  always  in 
certain  fixed  or  definite  proportions ;  that  is,  the  same  com- 
pound substance  always  contains  the  same  elements  combined 
in  a  constant  proportion :  thus  water,  whatever  may  be  its 
quantity,  or  however  generated,  consists  of  8  parts  of  oxygen 
to  1  part  by  weight  of  hydrogen  :  thus  1  part  of  hydrogen, 
combines  with  16  parts  by  weight  of  sulphurate  form  sul-> 
phuretted  hydrogen :  thus  1  part  of  hydrogen  combines  with 
6  parts  by  weight  of  carbon,  to  form  carburetted  hydrogen, 
(olefiant  gas.)  The  numbers  representing  the  proportional 
weights  in  which  bodies  combine  are  called  their  chemical 
equivalents. 

Taking  1  as  the  combining  equivalent  of  hydrogen,  8  will 
be  the  combining  equivalent  of  oxygen,  16  that  of  sulphur, 
and  6  that  of  carbon.  Moreover,  while  8  and  6  represent 
the  proportional  numbers  in  which  oxygen  and  carbon  re- 
spectively combine  with  hydrogen,  these  numbers  also  repre- 
sent the  proportion  in  which  oxygen  and  carbon  combine 
with  each  other  or  with  any  other  substances :  thus  8  parts  of 
oxygen  combine  with  6  parts  by  weight  of  carbon,  to  form 
carbonic  oxide.  But  this  is  not  all :  when  the  same  bodies 
combine  in  more  than  one  proportion,  the  proportional  num- 
bers representing  each  successive  compound  are  multiples 
(or,  it  may  be,  submultiples)  of  those  in  the  first  compound. 
This  law  is  exhibited  in  the  following  examples  :  — 

Compounds -of  Carbon  and  Oxygen. 

Proportion  Proportion 

of  Carbon.  of  Oxygen.* 

Carbonic  oxide        -         -         -         6  -f-  8       =       14 

Carbonic  acid  6  -f-        16      =      22 


478 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


Compounds  of  Nitrogen  and  Oxygen. 


Protoxide  of  nitrogen 
Binoxide  of  nitrogen 
Hyponitrous  acid    - 
Nitrous  acid  - 
Nitric  acid 


Proportion 

Proportion 

of  Nitrogen. 

of  Oxygen. 

-       14         -f- 

8          : 

-     "U        ? 

16          : 

-       14         -f 

24          : 

14         -f 

32          = 

14         -|- 

40          = 

22 
30 
38 
46 
54 


The  equivalent  of  a  compound  body  is  the  sum  of  the 
equivalents  of  its  elements  :  thus  the  equivalent  of  carbonic 
oxide  is  14,  this  number  being  the  sum  of  6  and  8  ;  thus  the 
equivalent  of  nitric  acid  is  54,  this  number  being  the  sum  of 
14  and  40.  . 

39.  The  following  table  contains  a  list  of  the  names  of  the 
elementary  substances,  so  far  as  the}'  are  at  present  known, 
with  their  symbols  and  combining  equivalents.  Those  sub- 
stances printed  in  Italics  are  rare,  and  of  comparatively  little 
importance. 

Table  of  Equivalents  and  Symbols  of  62  Simple  Substances. 

Sym.  Name. 

H.  Hydrogen 

0.  Oxygen     - 
N.  Nitrogen  - 
Cl.  Chlorine    - 
C.  Carbon      - 

1.  Iodine 

S.  Sulphur     - 

P.  Phosphorus 

F.  Fluorine    - 

Br.  Bromine    - 

B.  Boron 

Se.  Selenium  - 


METALS. 


K.  Potassium 
Na.  '  Sodium  - 
Lithium  - 
Calcium  - 


L. 

Ca. 


Mg.      Magnesium 
Si.        Silicon 


Equiv. 

Sym. 

Name. 

1 

Al. 

Aluminum 

8 

Fe. 

Iron 

14 

Cu. 

Copper     - 

36 

Pb. 

Lead 

6 

Zn. 

Zinc 

126 

Cr. 

Crome 

16 

Hg. 

Mercury  - 

16 

Ag. 

Silver      - 

18 

Au. 

Gold 

78 

PI. 

Platinum 

10 

Sn. 

Tin 

40 

Co. 

Cobalt      - 

Mn. 

Manganese 

Ni. 

Nickel      - 

40 

Ba. 

Barium    - 

24 

Sr. 

Strontium 

6 

As. 

Arsenic    - 

20 

Sb. 

Antimony 

12 

Bi. 

Bismuth 

8 

Te. 

Tellurium 

Equiv. 
14 
28 
32 
104 
33 
28 
100 
108 
100 
98 
58 
30 
28 
30 
68 
44 
76 
128 
108 
64 


EXPERIMENTAL    CHEMISTRY.  479 


Sym.               Name.  Equiv. 

V.  Vanadium  -  -       68. 

U.  Uranium  -  -       60 

Mo.  Molybdenum  -  -      48 

Tn.  Tungsten  -  -       94 

Ti.  Titanium  -  -      24 

Cm.  Columbium  -  -     184 

Nr.  Niobium  -  -         ? 

Pe.  Pelopium  -  -         ? 

No.  Norium  ? 

G.  ^Glucinum  26 

Zr.  Zirconium  34 

Th.  Thorium  -  60 


Sym.                Name.  Equiv. 

D.  Didymium  ? 
Ln.  Lanthanium  '  -         -       48 
Ce.  Cerium    -  -                 46 
Y.  Yttrium  -         -       32 
Tb.  'Terbium  ? 

E,  Erbium  ? 
Cd.  Cadmium  -        -       56 
Pd.  Palladium  54 
R,  Rhodium  -         -       52 
Os.  Osmium  -         -     100 
Ir.  Iridium  98 
Ru.  Ruthenium  -                52 


The  arrangement  of  the  elementary  substances,  in  this  table,  is  merely 
adopted  to  suit  the  order  observed  in  the  other  portions  of  this  work. 

40.  The  first  letter  or  letters  of  the  Latin  name  of  a  sim- 
ple substance  is  taken  as  its  symbol ;  and  the  symbol  of  any 
substance  always  represents  its  combining  equivalent.  Thus 
O  stands  for  one  equivalent  of  oxygen ;  2O,  or  O2,  stands  for 
two  equivalents  of  oxygen,  and  so  on.  Compounds  are 
expressed  by  the  equivalents  of  simple  substances  which 
enter  into  their  composition :  thus  sulphuric  acid  is  composed 
of  one  equivalent  of  sulphur  and  three  equivalents  of  oxygen  : 
the  symbol  of  sulphuric  acid,  therefore,  is  S  +  O3,  or  more 
simply  SO3,  and  the  combining  equivalent  =.  16  +  3  X 
8  =  40. 

The  sign  of  equality  (=)  is  used  to  express  an  identity  of 
composition,  but  not  always  an  identity  in  the  form  of  the 
arrangement  of  the  elements. 

The  names  given  to  compound  substances  are  such  as  to 
indicate  their  elementary  composition. 

Compounds  containing  oxygen  are  called  acids  or  oxides, 
according  as  they  do  or  do  not  possess  acidity.  Thus  an  oxide 
of  iron .  contains  oxygen  and  iron.  The  termination  ic  is 
placed  to  the  name  of  a  substance  when  it  becomes  an  acid : 
thus  we  have  sulphuric  acid,  which  is  a  compound  of  sulphur 
and  oxygen.  When  the  substance  forms  two  acids,  that  which 
contains  the  smallest  portion  of  oxygen  terminates  in  ous ; 


480  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

thus  we  have  sulphurous  acid.  The  termination  uret  also 
indicates  the  combination  of  a  variety  of  substances :  thus  we 
have  sulphuret  of  iron,  which  expresses  a  compound  of  sul- 
phur and  iron.  Degrees  of  oxidation  are  sometimes  expressed 
by  Greek  or  Latin  prefixes :  thus  protoxide  expresses  the 
first  degree  of  oxidation,  frmoxide  the  second,  Peroxide  the 
third,  and  so  on.  The  highest  degree  of  oxidation  is  usually 
expressed  peroxide.  "When  an  acid,  whose  name  ends  with 
an  z'c,  forms  a  salt,  its  name  terminates  with  an  ate  •  thus 
nitric  acid  forms  a  nitrate  ;  while  a  su^phurows  acid  forms  a 
sulphite  ;  and  so  on  to  other  cases. 

EXERCISES  ON  THE  USE  OP  CHEMICAL  FORMULAE. 

41.  One  of  the  greatest  advantages  of  chemical  formulae 
is,  that  they  enable  us  to  represent  chemical  combinations  and 
changes  with  such  clearness  and  precision. 

1.  1  eq.*  water  =  1  eq.  hydrogen  -f-  1  eq.  oxygen 

=  H  +  O,  or  HO, 
=  l-|-8=9. 

2.  1  eq.  carbonic  acid  =  1  eq.  carbon  -j-  2  eq.  oxygen 

=  C  -f  02,  or  C02, 
•=6  +2  X  8  =  22. 

3.  1  eq.  nitric  acid  =  1  eq.  nitrogen  +  5  eq.  oxygen 

—  N  -f  O5,  or  NO5, 
=  14  -f-  4  X  8  =  54. 

4.  1  eq.  potassa  =  1  eq,  potassium  -f-  1  eq.  oxygen 

=  K  +  O,  or  KO, 

=  40  +  8  =  48. 

5.  1  eq.  carbonate  of  potassa  =  1  eq.  potassa  -f  1  eq.  carbonic  acid 

=  KO  -f  CO2,  or 

KO  CO2, 
=  48  +  22  =  70. 

6.  1  eq.  ammonia  =  1  eq  nitrogen  -f-  3  eq.  hydrogen 

=  N  +  H3,  or  NH3, 
=  14  +  3  X  1  =  17. 

*  Eq.  is  used  as  an  abbreviation  of  the  word  equivalent. 


EXPERIMENTAL    CHEMISTRY.  481 

7.  1  eq.  hydrochloric  acid  =  1  eq.  hydrogen  -f-  1  eq.  chlorine 

=  H  4-  Cl,  or  HC1, 
=  1  4-  36  =  37. 

8.  1  eq.  hydrochlorate  of  ammonia  =  NH3  4-  HC1 

=  17  +  37  =  54. 

9.  1  eq.  bichloride  of  platinum  =  PI  -f-  2C1,  or  P1C12, 

=  98  -j-  2  X  &  =  170. 

10.  1  eq.  lime  =  1  eq.  calcium  -}-  1  eq.  oxygen 

=  Ca  -f  O,  or  CaO, 
=  20  -f  8  =  28. 

11.  1  eq.  carbonate  of  lime  =  CaO  -f-  CO2,  or  CaO  CO2, 

=  28  4-  22  =  50. 

12.  The  action  in  Exp.  3,  Art.  12,  is  as  follows  :  — 

1  eq.  carbonate  of  potassa  -f  1  eq=  lime 
=  KO  CO2  +  CaO 
=  KO  +  CaO  C02. 

Here  KO,  or  potassa,  remains  in  solution,  and  CaO  CO2,  or  carbonate 
of  lime,  is  precipitated. 

13.  -The  action  in  Exp.  1,  Art.  14,  is  as  follows  :  — 

1  eq.  sulphuric  acid  -f-  1  eq.  water  -}-  1  eq.  zinc 
=  SO3  -f  HO  -j-  Zn 
==  ZnO  S03  4-  H. 

Here  ZnO  SO3,  or  sulphate  of  oxide  of  zinc,  remains  in  solution,  and 
the  hydrogen  is  given  oif. 

14.  The  action  in  Exp.  1,  Art   13,  is  as  follows  :  — 

1  eq.  hydrochloric  acid  -f-  1  eq.  carbonate  of  lime 
=  HC1  -f  CaO  C02 
=  CaCl  +  HO  4-  CO2. 

Here  CaCl  4-  HO,  or  chloride  of  calcium  with  water,  is  formed,  and 
CO2,  or  carbonic  acid,  is  given  off. 

15.  The  action  in  Exp.  1,  Art.  26,  is  as  follows  :  — 

1  eq.  sulphate  of  magnesia  4-  1  eq.  carbonate  of  potassa 
=  MgO  SO3  4-  KO  CO2 
=  KO  S03  4-  MgO  C02 

Here  MgO  CO2,  or  carbonate  of  magnesia,  falls,  and  KO  SO3,  or 
sulphate  of  potassa,  remains  in  solution. 
41 


482          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

16.  1  eq.  nitrate  of  copper,  or  nitrate  of  the  oxide  of  copper, 

=  1  eq.  oxide  of  copper  -|-  1  eq.  nitric  acid 
=  CuO.-f  NOS,  or  CuO  NO3. 

17.  The  action  in  Exp.  3,  Art.  35,  is  as  follows  :  — 

1  eq.  nitrate  of  the  oxide  of  silver  -{-  1  e(l'  hydrochloric  acid 
=  AgO  N05  +  HC1 
=  N05'+IIO-f-AgCl. 

Here  NO5,  or  nitric  acid  with  water,  remains  in  solution,  and  AgCl, 
or  chloride  of  silver,  falls. 


SECTION  V. 

EXPERIMENTS    CONDUCTED    ON   A  LARGER    SCALE,  OR  WITH 
A    MORE    COMPLETE    APPARATUS. 

OXYGEN O. 

42.  Preparation.  —  To  obtain  oxygen  pure,  the  substance  which  sup- 
plies it  is  placed  in  a  retort  or  tube,  and  exposed  to  heat ;  when  %as  is 

"evolved,  it  must  be  collected  over  water,  either  in  the  pneumatic  trough 
or  in  a  gas  holder.     (See  pages  463  and  464.) 

43.  Oxygen  obtained  from  black  oxide  of  manganese,  — This  substance 
is  used  by  itself  when  large  quantities  of  the  gas  are  required.     The 
oxide  is  introduced  into  an  iron  bottle,  to  the  mouth  of  which  an  iron 


Fig.  21. 

tube  a,  is  adapted,  and  luted  or  plastered  over  with  common  pipe  clay, 
made  into  paste  with  water  The  extremity  of  this  tube  is  luted  to  a 
flexible  tube  6,  the  outer  end  of  which  is  inserted  into  the  water  of  a 


EXPERIMENTAL    CHEMISTRY.  483 

gas  holder.  The  bottle  is  placed  upon  a  good  fire.  When  the  manga- 
nese attains  a  red  heat,  it  gives  off  a  portion  of  its  oxygen,  which  rises 
within  the  ga&  holder. 

The  chemical  changes  which  take  place  in  this  process  are  exhibited 
in  the  following  formulae  :  — 

3  eq.  binoxide  of  manganese 
=  2MivO2  =  Mn2O4  =  Mn2O3  +  O. 

Here,  after  the  process  is  completed,  Mn2O3,  or  sesquioxide  of  manga- 
nese, remains  in  the  retort.  The  gas  should  stand  for  a  short  time  over 
water,  in  order  to  absorb  any  carbonic  acid  gas  which  it  may  contain. 

44.    Oxygen  obtained  from  chlorate  of  potassa.  —  Mix  about  equal 
parts  of  chlorate  of  potassa  and  black  oxide  of  manganese  in  a  mortar  ; 
introduce  the  mixture  into  a  small  copper  or  green  glass  retort ;  apply 
the  flame  of  a  spirit  lamp, 
and  receive  the  gas  in  the 
gas  holder,  or  the  pneumatic 
trough. 

If  only  a  small  quantity 
of  the  gas  is  wanted,  the 
mixture  may  be  introduced 
into  a  large  test  tube,  hav- 
ing a  cork  perforated  by  a  Fig.  22. 
bent  exit  tube,  as  in  the  annexed  cut. 

The  decomposition  is  represented  by  the  following  formulae  :  — 

1  eq.  chlorate  of  potassa 
=  1  eq.  potassa  -f-  1  eq.  chloric  acid 
=  KO  -f-  C1O5  =  KC1  -f  O6. 

Here  the  heat  resolves  the  chlorate  of  potassa  into  KC1,  or  chloride  of 
potassium,  and  O&,  or  6  eqs.  of  oxygen.  The  manganese  in  the  mixture 
merely  aids  in  keeping  a  steady  heat. 

EXPERIMENTS  WITH  OXYGEN. 

1.  Introduce  a  lighted  taper  into  a  bottle  of  this  gas :  the  flame  is 
increased  in  size  and  brilliancy  Introduce  a  candle  with  a  wick  red  : 
it  bursts  into  flame.  (See  Fig.  23.) 

2  Put  some  pounded  charcoal  on  a  cup  attached  to  a  wire  passing 
through  a  cork;  heat  the  charcoal,  over  a  spirit  lamp,  to  redness; 
plunge  it  into  the  gas  :  the  charcoal  glows  with  great  brightness,  and 
bursts  into  flame.  Here  the  product  of  combustion  is  carbonic  acid  gas. 
(See  Fig.  24.) 

3.  Burn  phosphorus  in  the  same  manner :  it  burns  with  great  splendor. 


484          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


4.  Burn  sulphur  in  the  same  manner  :  it  burns  with  a  beautiful  blue 
flame. 

5.  Roll  a  piece  of  fine  steel  wire  in  a  spiral  form  round  a  glass  tube  : 
fix  one  extremity  of  the  wire  in  a  cork,  and  to  the  other  extremity 


Fig.  23 


Fig.  24. 


Fig.  25. 


attach  a  piece  of  cotton  wick  dipped  in  melted  sulphur  ;  ignite  the  wick, 
and  plunge  it  into  a  bottle  of  oxygen  :  the  wire  takes  fire,  and  burns 
with  beautiful  scintillations.  The  product  of  combustion  hi  this  case  is 
oxide  of  iron.  To  prevent  the  bottle  from  breaking,  the  bottom  should 
be  covered  with  sand.  (See  Fig.  25.) 

6.  Introduce  some  dark-colored 
venous  blood  into  a  bottle  of  oxy- 
gen :  the  blood,  upon  being  sha- 
ken, soon  acquires  the  florid  color 
of  arterial  blood. 

7.  Project  the  oxygen  from  a 
gas  holder  on  the  flame  of  a  spirit 
lamp :    great    heat  is   produced. 
Hold  a  small  piece  of  ch^t  or 

lime  before  the  flame :  the  light  Fig.  26. 

of  the   lime  is  brilliantly  white 

and  intense.     Burn  a  piece  of  watch  spring  in  the  flame  ;  &c. 

8.  Take  a  large  piece  of  charcoal,  and  make  a  small  hole  in  it ;  hold 
this  part  of  the  charcoal  over  a  lamp  until  it  becomes  red  hot ;  drop  a 
small  cast  iron  nail  into  the  hole ;  hold  the  heated  charcoal  before  a 
stream  of  oxygen  (issuing  from  a  jet  with  its  orifice  turned  downwards  :) 
the  charcoal  burns  rapidly  ;  the  nail  becomes  white  hot,  then  fuses,  and 
finally  burns,  giving  off  a  brilliant  shower  of  ignited  sparks  of  the  metal. 
This  is  one  of  the  most  beautiful  experiments  in  the  whole  range  of 
chemistry. 

Various  other  metals  may  be  ignited  in  the  same  manner. 
The  common  mouth  blowpipe.  —  This  consists  of  a  brass  tube,  having  a 
very  small  orifice  or  jet  at  one  end,  for  projecting  a  small  constant  stream 


EXPERIMENTAL    CHEMISTRY. 


485 


10 


of  air  upon  the  flame.     In  the  flame  of  a  common  candle,  &  is  a  hollow 
cone  containing  combustible  gases  in  excess;    this  is  surrounded  by  a 
sheet  of  flame  «,  Vhere  the  combustible  material  is  in  contact  with  the 
oxygen   of   the    air.      When  we 
blow  through  this  flame,  by  means 
of  the  blowpipe,  the  circumstances 
are  completely  changed  ;  b,  in  the 
second  figure,  contains  a  powerful 
flame,    having    combustible   gases 
in   excess ;  this  portion  is  called 
the  deoxidizing  01 'reducing  flame, 
for  it  deprives  substances  of  their 
oxygen  ;  a,  in  the  second  figure,  Fig.  27. 

is  a  flame  where  the  oxygen  pre- 
ponderates ;  this  portion  is  called  the  oxidizing  flame,  for  it  communi- 
cates oxygen  to  the  substance  heated  in  it. 

When  a  metal  is  to  be  brought  to  the  state  of  an  oxide,  it  is  placed  in 
the  oxidizing  flame  a  ;  and  when  an  oxide  is  to  be  reduced  to  the  metal- 
lic state,  it  is  placed  in  the  deoxidizing  or  reducing  flame  b.  The  most 
powerful  heat  is  produced  at  the  apex  of  the  cone  c. 

In  using  the  mouth  blowpipe,  the  student  must  endeavor  to  acquire 
the  power  of  keeping  up  a  constant  and  steady  blast,  by  forming  his 
mouth  into  a  bag  of  air,  while  at  the  same  time  he  breathes  through  his 
nostrils.  The  manner  of  doing  this  is  difficult  to  explain :  by  repeated 
trials,  however,  he  will  see  that  it  is  possible  to  do  so. 

Experiment.  —  Place  one  or  two  grains  of  oxide  of  lead  (litharge)  on  a 
piece  of  charcoal,  and  hold  the  substance  in  the  reducing  flame  b ;  the 
metallic  lead  is  produced  in  the  form  of  a  brilliant  globule ;  bring  the 
globule  to  the  oxidizing  flame  a ;  the  metal  is  oxidized,  and  presents  a  dull 
appearance.  Various  other  metals  may  be  treated  in  the  same  manner. 

The  peculiar  action  of  the  flame  of  the  blowpipe  constitutes  a  most  in- 
teresting and  useful  department  of  experimental  chemistry. 

HYDROGEN. 

45.  Preparation.  —  This  gas  is  most  conveniently  prepared  from  zinc 
and  diluted  sulphuric  acid.  Put  some  zinc  cuttings, 
sulphuric  acid,  and  about  five  times  the  quantity  of 
water,  into  a  retort  r,  (see  Fig.  14,  page  463,)  or 
into  a  bottle  b,  with  the  bent  tube  t ;  great  heat  is  pro- 
duced by  the  mixture  of  the  acid  and  water,  and  the 
gas  is  copiously  evolved,  which  may  be  received  over 
water  in  the  pneumatic  trough  or  in  the  gas  holder. 
(See  Exercise  13,  Art.  41.)  Fi9-  28- 

41  * 


486 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


EXPERIMENTS  WITH  HYDROGEN. 

1.  Invert  a  jar  of  hydrogen  over  a  candle;  the* 
flame  of  the  candle  is  extinguished,  but  the  gas 
burns  at  the  mouth  of  the  vessel.    In  this  way  the 
gas  takes  some  time  before  it  is  burned  away. 

2.  Ignite  a  jar  or  bottle  of  hydrogen,  having  the 
mouth  of  the  vessel  uppermost ;  in  this  case  the  gas 
burns  much  more  quickly  away,  in  consequence  of 
its  great  lightness  as  compared  with  the  air. 

3.  Introduce  the  gas  into  a  jar  a,  having  a  gas 
burner  <7,  and  stop  cock  c ;  open  the  cock,  and  at  the 
same  time  depress  the  jar  in  the  water ;  ignite  the 
gas  as  it  issues  from  the  small  orifice  g ;  the  gas 
burns  with  a  pale  yellow  flame,  which  gives^  off  a 
great  deal  of  heat. 

Hold  a  dry  glass  tumbler  over  the  flame : 
•water  is  deposited. 

Repeat  Exp.  2,  Art.  14. 

4.  Mix  in  a  strong  bottle  1  measure  of  hy- 
drogen with  2£  or  3  measures  of  common  air ; 
apply  the  flame  of  a  candle  to  the  mouth  of  the 
bottle:   the  mixture  detonates  with  a  consid- 
erable report. 

5.  Fill  a  bladder  with  this  gas  from  a  capped 
receiver  at  the  pneumatic  trough,  as  exhibited 
in  the  annexed  cut,  or  from  the  gas  holder ; 
adjust  a  common  tobacco  pipe  to  the  stop  cock, 
and  blow  soap  bubbles  by  giving  a  gentle  pres- 
sure to  the  bladder  :  these  soap  bubbles,  being 
filled  writh  hydrogen,  are  lighter  than  the  air, 
and  they  ascend  in  the  atmosphere  like  little 
balloons.     Bring  the  flame  of  a  candle  in  con- 
tact with  one  of  these  hydrogen  bubbles  :  it 
explodes. 

6.  Fill  a  small  balloon  with  hydrogen,  or 
common  street  gas,  and  load  it  with  a  light 
paper  car,  so  as  to  keep  it  suspended  in  the  air. 

7.  Throw  a  stream  of  hydrogen  on  spongy 
platinum.     (See  Exp.  3,  Art.  37.)    To  in- 
sure the  success  of  the  experiment,  the  plat- 
inum should  be  previously  heated  to  redness 
before  the  spirit  lamp. 

8.  Mix  over  the  pneumatic  trough  a  por- 
tion of  oxygen  with  twice  its  volume  of  hy- 


Fig.  29. 


Fig.  31. 


32. 


EXPERIMENTAL    CHEMISTRY. 


487 


drogen  :  fill  a  common  soda  water  bottle  full  with  the  mixed  gases ;  ap- 
ply the  flame  of  a  taper  :  the  gases  detonate  with  a  loud  report. 


Fig.  33. 


Composition  of  Water. 

46.  It  has  already  been  explained  that  water  is  composed  of  8  parts 
by  weight  of  oxygen  and  1  part  of  hydrogen.     Now,  oxygen  is  exactly 
16  times  heavier  than  hydrogen ;  hence  it  follows  that  there  must  be 
double  the  quantity  by  volume  of  hydrogen  to  form  water.     The  com- 
position of  water  may  be  determined  in  two  ways :  first,  by  synthe- 
sis, or  by  bringing  the  elements  together ;  second,  by  analysis,  or  by 
separating  the  elements  from  each  other. 

Synthesis.  —  Introduce  the  mixed 
gases,  2  volumes  or  measures  of  hy- 
drogen and  1  volume  or  measure  of 
oxygen,  into  a  strong  graduated  tube 
(Volta's  Eudimometer}  having  two 
wires  nearly  meeting  each  other 
within  the  tube  at  the  top  ;  pass  an 
electric  spark  through  the  mixed 
gases  by  means  of  a  charged  Leyden 
jar,  as  shown  in  the  annexed  cut : 
the  gases  combine  with  ignition,  wa- 
ter is  formed,  and  a  complete  vacuum 
is  produced,  which  is  filled  up  by  the  ascent  of  the  water  in  the  trough. 

Analysis.  —  Two  equal  tubes,  O  andH,  filled 
with  water,  are  inverted  over  the  two  poles  of 
a  galvanic  battery ;  when  the  battery  is  put  in 
action  the  water  is  resolved  into  the  two  gases  ; 
the  oxygen  rises  in  the  tube  O  placed  over  the 
positive  pole,  and  the  hydrogen  into  the  tube 
H  placed  over  the  negative  pole.  As  the  analy- 
sis proceeds,  it  will  be  seen  that  the  volume  of 
the  hydrogen  is  always  double  that  of  the 
oxygen. 

47.  Water  is  also  decomposed  by  passing  a 
current  of  steam  through  an  iron  tube  partial- 
ly filled  with  iron  filings,  and  kept  at  a  red  Fig.  34. 
heat  by  a  furnace.     In  this  cut,  r  represents 

the  retort  in  which  the  water  is  being  boiled  ;  t  t  the  red  hot  tube  passing 
through  the  furnace  F :  the  bent  pipe  b  conveys  the  hydrogen  gas  into  a 
jar  e  standing  on  the  shelf  of  a  pneumatic  trough  T.  In  this  interesting 
experiment  the  steam  passing  over  the  heated  iron  is  decomposed ;  the 


488 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


iron,  taking  up  the  oxygen,  becomes  an  oxide  of  iron,  and  the  hydrogen 
is  disengaged. 


Fig.  35. 

Ignition  of  the  Mixed  Gases.      Oxy-hydrogen  Blowpipe. 

48.  The  simplest  manner  of  showing  the  intense  heat  generated  by 
the  ignition  of  mixed  gases  is  as  follows : 

The  hydrogen  is  formed  in  the  bottle  5,  the 
cork  of  which  is  perforated  by  two  tubes ;  a 
is  a  funnel-shaped  tube,  for  the  purpose  of 
supplying  sulphuric  acid  as  it  is  required ;  t 
is  a  bent  tube  conveying  the  hydrogen  for 
ignition  ;  *  r  is  the  tube  with  a  stop  cock 
and  jet,  conveying  a  stream  of  oxygen  from 
a  gas  holder  on  the  hydrogen  flame.  The 
various  experiments  described  in  Art.  44 
may  be  tried  with  this  flame. 

49.  Daniel's  blowpipe.  —  In  this  appara- 
tus a  common  tube  b  receives  the  two  gases  contained  in  the  bladders  H 


Fig.  36. 


Fig.  37. 

and  O,  provided  with  stop  cocks.  The  hydrogen  is  first  ignited,  and 
then  the  pressure  upon  the  bladder  containing  the  oxygen  is  regulated 
so  as  to  produce  the  maximum  heating  effect  on  the  flame. 

*  This  is  an  excellent  arrangement  for  making  hydrogen,  as  well  as  sul- 
phuretted hydrogen,  when  largo  quantities  aro  required. 


EXPERIMENTAL    CHEMISTRY. 


489 


Fig.  38. 


50.  The  following  economical  apparatus,  answering  the  double  pur- 
pose of  a  gas  holder  -and  an  oxy-hydrogen  blowpipe,  has  been  success- 
fully used  by  the  author  of  this  work.     In  the 

annexed  cut,  r  represents  the  gas  receiver;  s 
a  shelf  which  screws  on  at  b;  e  and  c  stop 
cocks,  as  in  the  ordinary  gas  holder,  (seep.  464  ;) 
h  g  a  jet  filled  with  wire  gauze  at  the  thick  part 
h, ;  p  an  aperture  for  receiving  the  beak  of  a  re- 
tort ;  n  k  a  bent  pipe  passing  into  the  interior 
of  the  receiver  r;  k  t  a  flexible  tube,  which 
screws  on  at  k,  and  forms  a  connection  with  a 
bladder  containing  the  mixed  gases ;  /  a  funnel 
with  a  tube  and  stop  cock  for  screwing  on  at 
k.  The  tubular  portion  d  b  is  about  |  inch  in- 
ternal diameter  and  2  inches  in  length.  When 
the  apparatus  is  to  be  used  as  an  oxy-hydrogen 
blowpipe,  water  is  introduced  into  the  receiver,  so  as  to  stand  at  the  level 
d;  the  orifice  at  b  is  closed  by  means  of  a  cork,  and  the  bladder  con- 
taining the  mixed  gases  is  screwed  on  at  k;  the  safety  jet  h  g  is  screwed 
on  the  stop  cock  e ;  upon  pressure  being  applied  to  the  bladder,  the 
mixed  gases  rise  through  the  water,  and  filling  the  space  d  b,  pass 
out  in  a  strong  stream  through  the  jet  g,  and  are  there  ignited.  This 
arrangement  is  perfectly  safe,  for  in  the  event  of  the  flame  passing 
along  the  safety  tube  A,  we  can  only  have  an  explosion  of  the  gases  con- 
tained in  the  small  chamber  d  b,  the  only  effect  of  which  would  be  to 
blow  out  the -cork  in  the  orifice  6,  as  the  large  body  of  water  in  r  most 
effectually  cuts  off  all  communication  with  the  gases  in  the  bladder. 
When  the  apparatus  is  to  be  used  as  a  gas  holder,  the  shelf  s  is  screwed 
on  at  b,  the  funnel  fat  k,  and  gases  are  received  and  transmitted  in  the 
same  manner  as  in  the  ordinary  gas  holder. 

Analysis  of  Atmospheric  Air  ly  the  Detonation  of  Hydrogen 
in  VoltcCs  Eudiometer. 

51.  Mix  over  the  pneumatic  trough  2  volumes  of  atmospheric  air  and 
1  volume  of  hydrogen ;  introduce  a  small  portion  of  this  mixture  into 
the  eudiometer  tube  (Art.  46)  so  as  to  occupy  15  div^ons  of  the  tube; 
detonate  by  the  electric  spark  :  after  detonation  the  gas  only  occupies  9 
divisions  of  the  tube ;  that  is,  6  parts  have  disappeared,  in  consequence 
of  all  the  oxygen  having  combined  with  a  portion  of  the  hydrogen  to 
form  water.     Now,  the  gaseous  mixture  in  the  tube  contained  10  parts 
of  air  and  5  of  hydrogen ;  and  since  water  is  composed  of  1  volume  of 
oxygen  and  2  volumes  of  hydrogen,  one  third  of  the  diminution  must 


490  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

give  the  quantity  of  oxygen  in  the  10  volumes  of  air  originally  in  the 
tube ;  that  is,  2  volumes  of  oxygen  have  disappeared  ;  but  2  is  one  fifth 
of  10  ;  therefore  one  fifth  of  atmospheric  air  is  oxygen,  and  the  remaining 
four  fifths  are  nitrogen  :  hence  in  100  volumes  of  air,  20  are  oxygen  and 
80  are  nitrogen. 

NITROGEN   AND    ITS    COMPOUNDS    WITH    OXYGEN. 

52.   For  the  .preparation  and  properties  of  nitrogen,  see  Art.  15,  and 

Exps.  4  and  5. 

* 

Protoxide  of  Nitrogen  —  NO. 

This  gaseous  compound  is  familiarly  known  by  the  name  of  the  laugh- 
ing gas,  from  the  ludicrous  effect  which  it  has  upon  persons  who  respire 
it.  This  gas  is  not  inflammable,  but  it  supports  combustion  with  greater 
brilliancy  than  common  air. 

Preparation.  —  Introduce  some  crystals  of  nitrate  of  ammonia  *  'into 
a  large  retort ;  apply  the  heat  of  an  Argand  lamp  having  a  copper  flue, 
to  give  steadiness  to  the  flame  :  at  a  temperature  of  400°  the  salt  fuses, 
and  then  gives  off  the  gas  in  great  abundance,  which  may  be  received  in 
the  gas  holder  filled  with  warm  water,  as  cold  water  largely  absorbs  the 
gas.  It  should  stand  for  two  or  three  hours  over  a  little  water,  to  ab- 
sorb any  fumes  of  nitrous  acid  that  may  be  formed  in  the  process.  The 
whole  of  the  salt  is  resolved  by  heat  into  this  gas  and  water,  as  shown 
by  the  following  symbols :  — 

1  eq.  nitrate  of  ammonia 
=  1  eq.  ammonia  -|-  1  eq.  nitric  acid 
=  NH3  +  N05 
=  H3  -f  03  +  N2+  02  =  3HO  +  2NO. 

Hence  it  appears  that  1  eq.  of  nitrate  of  ammonia  yields  3  eq.  of  wa- 
ter and  2  eq.  of  the  protoxide  of  nitrogen. 

EXPERIMENTS. 

1.  Plunge  a  burning  candle  into  a  bottle  of  this  gas :  the  flame  is 
much  increased  in  brilliancy  in  consequence  of  the  large  quantity  of 
oxygen  which  th^as  contains. 

*  To  prepare  this  salt,  add  carbonate  of  ammonia  in  powder  to  nitric  acid 
diluted  with  about  three  parts  of  water  until  effervescence  ceases ;  evaporate 
the  solution  until  a  drop  of  the  liquid  let  fall  upon  a  cold  plate  becomes  a 
solid  mass.  A  little  ammonia  should  be  added  towards  the  close  of  the.  pro- 
cess to  render  the  salt  perfectly  alkaline. 


EXPERIMENTAL    CHEMISTRY. 

2.  Introduce  a  large  splinter  of  wood  having  a  glowing  red  spark  into 
this  gas  :  the  flame  is  rekindled,  as  in  the  case  of  oxygen  gas. 

3.  Transfer  this  gas  from  the  gas  holder  into  a  damp  bladder  having 
a  wide  wooden  mouth  piece  ;  place  the  mouth- 
piece between  the  teeth  of  the  person  who  is 

to   inhale  the  gas  ;    let  him  close  his  nostrils 

with  his  fore  finger  and  thumb,  and  then  let 

him  breathe  the  gas  in  the  bladder:  various- 

effects,  more  or  less   ludicrous,    are  produced  Fig.  39. 

upon  persons  inhaling  the  gas.     All  kinds  of 

apparatus  should  be  removed,  for  they  are  liable  to  be  injured  by  the 

inhaler ;  or  better,  let  the  inhaler  be  out  doors. 

Binoxide  of  Nitrogen  —  NO2. 

This  compound  is  a  colorless  gas,  similar  in  appearance  to  common 
air  :  it  is  sparingly  absorbed  by  water. 

Preparation.  —  Put  some  copper  cuttings  into  a  retort,  pour  nitric 
acid  upon  them,  and  then  add  about  an  equal  quantity  of  water :  *  brisk 
effervescence  takes  place  without  the  aid  of  heat,  and  the  gas  may  be 
collected  over  water  in  the  pneumatic  trough. 

The  decomposition  is  represented  by  the  following  formulae :  — 

4  eq.  nitric  acid  -f-  3  eq.  copper 
=  4NO3      -f  3Cu 
=  NO2O3  +  3NO5  +  3Cu 
=  N02       +  3(CuO  -f  N05) 
=  binoxide  of  nitrogen  -f-  3  nitrate  of  the  oxide  of  copper. 

EXPERIMENTS. 

1.  Transfer  a  bottle  of  this  gas  over  the  pneumatic  trough  into  a 
similar  bottle  nearly  filled  with  common  air  ;  red  fumes  of  nitrous  acid 
(NO4)  are  instantly  formed,  which  are  soon   absorbed  by  the  water. 
This  constitutes  a  characteristic  property  of  the  binoxide  of  nitrogen, 
and  it  is  used  in  this  way  to  detect  the  presence  of  free  oxygen. 

2.  Plunge  a  piece  of  burning  phosphorus  into  a  bottle  of  this  gas : 
the  phosphorus  continues  to  burn. 

3.  Burn  a  mixture  of  this  gas  and  hydrogen,  (see  cut  to  Exp.  3,  Art. 
45  :)  the  mixed  gases  burn  with  a  green-colored  flame. 

4.  Transfer  a  bottle  of  binoxide  of  nitrogen  to  a  cup  containing  a  so- 
lution of  sulphate  of  iron  :  the  solution,  becomes  black. 

*  The  diluted  acid  should  have  a  specific  gravity  of  12. 


492 


NATURAL   AND    EXPERIMENTAL    PHILOSOPHY. 


Nitric  Acid — NO5. 

For  the  leading  properties  of  this  acid,  see  Art.  17. 

Preparation.  —  Mix  equal  weights  of  nitrate  of  potassa  (nitre)  and  oil 
of  vitriol  of  commerce  in  a  retort ;  heat  the  retort  over  a  chauffer  a, 
containing  heated  charcoal,  (a  sand  bath  or  an  Argand  lamp  would  an- 
swer the  purpose  equally  as  well :)  nitric  acid  distils  over,  and  is  con- 


Fig.  40. 

densed  in  the  liquid  form  in  the  receiver  c,  kept  cool  by  a  stream  of 
water  proceeding  from  a  jar  d.    The  stream  of  water  may  be  conveniently 
supplied  from  a  funnel  having  its  tube  partially  closed  by  a  piece  of  rag. 
The  decomposition  is  as  follows :  — 
2  eq.  sulphuric  acid  -{-  water  -f-  1  eq«  nitrate  of  potassa 
=  S03  +  2HO  +  KO  N05 
=  KO  2SO3  +  HO  4-  NO5  4-  HO 
=  bisulphate  of  potassa  and  water  4- 
nitric  acid  and  water. 

Distillations  of  any  kind  are  conveniently  effected  by  means  of  Lie- 
big's  condensing  tube. 


41. 


The  liquid  to  be  distilled  is  placed  in  the  retort  r,  to  which  a  sufficient 
heat  is  applied  to  boil  the  liquid  :  the  vapor,  as  it  passes  along  the  tubes, 


EXPERIMENTAL    CHEMISTRY.  493 

is  condensed  by  the  cold  kept  up  in  the  condensing  tube  S  t  F,  and  the 
liquid  drops  into  the  receiver  R.  The  construction  of  this  condensing 
tube  is  exceedingly  ingenious  :  t  is  a  wide  tin  tube  ;  F  a  funnel  passing 
into  it  for  the  purpose  of  supplying  cold  water  ;  S  a  siphon  for  carrying 
off  the  hot  water ;  a  gfass  tube  passing  through  this  tin  tube  is  connected 
with  the  beak  of  the  retort  and  the  receiver  R;  this  glass  tube  passes 
through  perforated  corks  inserted  at  each  end  of  the  tin  tube  t.  Now, 
this  glass  tube  is  continually  surrounded  by  cold  water  ;  for  while  cold 
water  is  being  supplied  by  the  funnel  F,  the  water,  as  it  becomes  heated, 
rises  within  the  tin  tube,  and  is  carried  off  by  the  siphons. 

Experiment.  —  Heat  gently  some  oil  of  turpentine  in  a  porcelain  basin ; 
pour  suddenly  upon  it  a  mixture  of  one  part  of  sulphuric  acid  and  two 
parts  of  nitric  acid :  combustion  takes  place,  with  the  evolution  of  a 
dense  smoke.  In  order  to  avoid  accident,  the  mixed  acid  should  be 
poured  from  a  bottle  tied  to  the  end  of  a  stick. 

CARBON,    SULPHUR,    AND    PHOSPHORUS,    THEIR     COMPOUNDS 
WITH  OXYGEN  AND  HYDROGEN. 

Carbonic   Oxide  —  CO. 

53.  This  is  a  colorless  gas  ;  it  is  the  gas  that  burns  with  a  blue  flame 
at  the  top  of  a  coke  or  charcoal  fire. 

Preparation.  —  Mix  pounded  oxalic  acid  with  sulphuric  acid  in  a 
retort,  and  apply  heat :  carbonic  oxide  and  carbonic  acid  gases  are  given 
off,  which  may  be  received  over  the  pneumatic  trough.  By  allowing 
the  gases  to  stand  over  water  for  a  few  hours,  or  by  agitating  them  with 
lime  water,  the  carbonic  acid  gas  is  absorbed,  and  the  carbonic  oxide  is 
left  pure.  Oxalic  acid  may  be  regarded  as  a  compound  of  carbonic 
oxide  and  carbonic  acid  with  water  ;  thus :  — 

1  cq.  oxalic  acid  =  C2O3  -f-  water  =  CO  -f-  CO2  +  water.  Now, 
the  sulphuric  acid  combines  with  the  water,  and  sets  the  two  gases  free. 

Experiment.  —  Plunge  a  lighted  taper  into  a  bottle  of  this  gas :  the 
taper  is  extinguished,  but  the  gas  burns  at  the  mouth  of  the  bottle  with 
a  beautiful  blue  flame :  thus  carbonic  oxide  is  inflammable^  but  it  does 
not  support  combustion. 

Carbonic  Acid —  CO2. 

54.  Preparation.  —  Carbonic  acid  gas,  being  more  than  1£  times  heav- 
ier than  common  air,  may  be  prepared  sufficiently  pure  by  the  following 
process.     The  gas  is  generated  in  the  bottle  b  (see  Exp.  1,  Art.  13  ;)  a 
bent  tube  bed  passes  through  a  cork  b,  and  descends  to  the  bottom  of 

42 


494          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


the  open  bottle  d :  as  the  gas  enters  the  bottle 
d,  the  common  air  is  displaced.  *  A  little  expe- 
rience will  readily  enable  the  experimenter  to 
ascertain  when  the  bottle  is  filled  with  the  gas. 

When  this  gas  is  received  over  the  pneumatic 
trough,  the  water  should  be  warm  ;  for  carbonic 
acid  gas  is  largely  absorbed  by  cold  water. 

EXPERIMENTS. 


Fig.  42. 


Fig.  43. 


1.  Invert  a  jar  or  bottle  of  the  gas  over  a  burning  candle :  the  gas  by 
its  gravity  falls  upon  the  candle,  and  extinguishes  the 

flame. 

2.  Place  a  burning  candle  in  an  open  jar ;  take  a 
bottle  of  carbonic  acid  gas,  and  pour  it  into  the  jar  : 
the  flame  is  extinguished.     This  shows  that  carbonic 
acid  gas  is  much  heavier  than  'common  air. 

3.  Pour  some  lime  water  into  a  bottle  containing 
this   gas :    carbonate  of  lime  is  formed  :   shake  the 
liquid,  and  it  becomes  clear,  in  consequence  of  the 
carbonate  of  lime  being  soluble  in  an  excess  of  car- 
bonic acid.     la  this  way  lime  is  dissolved  in  spring 
water. 

4.  Add  a  little  water  to  a  bottle  of  the  gas ;  shake  the  bottle ;  the 
water  takes  up  the  gas,  and  acquires  decided  acid  properties  ;  add  a  little 
solution  of  litmus :  the  blue  is  changed  to  red. 

Carburetted  Hydrogen  —  CH.,. 

55.  This  gas  is  formed  in  marshes  and  stagnant  pools  ;  it  is  but  little 
more  than  half  the  weight  of  common  air ;  it  is  highly  inflammable,  and 
forms  the^re  damp  of  the  miners.  When  coal  is  heated  to  redness,  it  is 
resolved  into  tarry  matter,  and  certain  gaseous  compounds  of  carbon  and 
hydrogen,  containing  about  seventy  per  cent,  of  carburetted  hydrogen. 

EXPERIMENTS. 

1.  Invert  a  bottle  filled  with  water  in  a  stagnant  pool;  insert  a  fun- 
nel into  the  bottle  to  catch  the  gas  ;  stir  up  the  bottom  of  ^he  pool  with 
a  stick :  bubbles  of  carburetted  hydrogen  gas  rise,  which  are  easily 
received  through  the  funnel.  Ignite  the  gas  thus  obtained :  it  burns 
with  a  yellow  flame. 


*  Gases  which  are  lighter  than  the  air,  such 
received  in  bottles  with  their  mouths  inverted. 


ammoniacal  gas,  may  be 


EXPERIMENTAL    CHEMISTRY. 


495 


2.  Mix  1  measure  of  this  gas  with  7.  or  8  of  common  air,  in  a  bottle ; 
apply  the  flame  of  a  candle  :  the  gas  explodes  with  some  violence.     Mix 
1  measure  of  the  gas  with  3  or  4  measures  *bf  air,  and  ignite  the  gases  : 
they  burn  without  explosion. 

3.  Put  some  pounded  coal  into  a  test  tube,  fitted  with  a  cork  and  the 
stem  of  a  tobacco  pipe ;  apply  the  flame  of  a  spirit  lamp  :  gas  is  disen- 
gaged, which"  may  be  inflamed  as  it  issues  from  the  small  orifice  of  the 
pipe. 

4.  The  flame  of  a  candle  is  produced  by  the  ignition  of  carburetted 
hydrogen  gases.     Bring  one  extremity  of  a  tube, 

about  f  of  an  inch  in  diameter,  into  the  centre  of 
the  flame  of  a  candle  :  the  gases  rise  up  the  tube, 
and  may  be  ignited  as  they  escape  at  the  upper  end. 
This  experiment  also  shows  that  flame  is  hollow. 

5(5.  The  Davy  lamp, —  Carburetted  hydrogen 
occurs  in  coal  pits,  from  the  decomposition  of  the 
coal,  where  it  sometimes  explodes  by  coming  in 
contact  with  flame ;  and  thus  melancholy  accidents 
take  place.  The  Davy  lamp  is  designed  to  prevent 
these  explosions.  Fig.  44. 

Experiment.  —  Take  a  piece  of  fine  wire  gauze :  hold  it  across  the 
flame  of  a  lamp  ;  the  flame  does  not  pass  through  the  gauze.  Blow  out 
the  flame,  and  ignite  the  smoke  as  it  rises  through  the  gauze :  the  flame 
does  not  descend  below  the  gauze. 


Fig.  45.  .  Fig.  46. 

This  experiment  exhibits  the  principle  upon  which  the  Davy  lamp  is 
constructed  :  the  metal,  being  a  high  conductor  of  heat,  cools  down  the 
temperature  of  the  inflammable  matter  in  contact  with  it,  and  thereby 
extinguishes  the  flame  on  the  side  opposite  to1  the  burning  body.  The 
Davy  lamp  simply  consists  of  a  lamp  surrounded  by  wire  gauze  to  pre- 


496         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

vent  flame  extending  from  the  interior  of  the  lamp  to  the  adjacent  at- 
mosphere. 

Olefiant   Gas,  or  Heavy   Carburetted  Hydrogen  —  C±  H4. 

57.  This  gas,  owing  to  its  illuminating  power,  is  the  most  valuable 
constituent  of  street  gas.     It  contains  a  larger  quantity  of-  carbon  than 
the  light  carburetted  hydrogen.     Coal  gas,  when  well  prepared,  contains 
about  20  per  cent,  of  defiant  gas. 

Preparation.  —  Mix  one  part  of  alcohol  with  5  or  6  parts  of  sulphuric 
acid  in  a  retort ;  apply  the  heat  of  an  Argand  lamp :  the  gas  comes  over 
in  great  abundance,  which  may  be  received  over  water  in  the  pneumatic 
trough. 

EXPERIMENTS. 

1.  Plunge  a  lighted  candle  into  a  bottle  of  this  gas :  the  flame  of  the 
candle  is  extinguished  ;  but  the  gas  burns,  at  the  mouth  of  the  bottle, 
with  a  fine,  brilliant  flame. 

2.  Burn  this  gas  in  a  capped  receiver..    (See  cut  to  Exp.  3,  Art.  45.) 

3.  Mix  3  volumes  of  oxygen  with  1  volume  of  olefiant  gas  in  a 
strong  common  soda  water  bottle;  ignite  the  mixed  gases:  a  violent 
explosion  takes  place,  carbonic  acid  and  water  being  formed :  thus  we 
have 

olefiant  gas  and  oxygen 
=  C2H2  +  O6  =  2CO2  +  2HO. 

1.  Mix  2  measures  of  chlorine  with  1  measure  of  olefiant  gas  in  a 
bottle ;  introduce  a  lighted  candle :  the  gases  burn  with  a  red  flame, 
with  a  copious  deposition  of  lampblack,  thereby  showing  that  olefiant 
gas  contains  carbon. 

Sulphurous  Acid —  SO2. 

58.  This  acid  may  be  procured  in  a  pure  state  by  boiling  in  a  retort 
sulphuric  acid  with  copper  cuttings  :  the  gas  may  be  received  by  dis- 
placement, as  in  Art.  54.     The  action  is  represented  by  the  .following 
symbols :  — 

2  eq.  sulphuric  acid  -f-  1  eq.  copper 

=  2SO3  -f-'  CU 

=  S02  4-  CuO  S03 

=  sulphurous  acid  -f-  sulphate  of  oxide  of  copper. 

By  passing  a  current  of  the  gas  through  water,  a  solution  of  sulphur- 
ous acid  is  obtained.  It  unites  with  bases,  forming  sulphites.  The  gas 
is  used  in  bleaching  wroollens 


EXPERIMENTAL    CHEMISTRY. 


497 


Sulphuric  Acid  —  SO3. 

59.  This  valuable  acid  is  made  by  the  manufacturer  on  a  large  scale, 
by  burning  sulphur  in  a  furnace,  where  nitric  acid  is,  at  the  same  time, 
formed  by  the  decomposition  of  nitrate  of  soda  by  means  of  sulphuric 
acid  :  the  sulphurous  and  nitric  acids  pass  into  a  succession  of  leaden 
chambers  containing  a  portion  of  water,  to  dissolve  the  sulphuric  acid,  as 
it  is  being  formed  by  the  nitric  acid  giving  up  a  portion  of  its  oxygen. 
In  the  annexed  cut,  a  represents  the  furnace  in  which  the  sulphurous 


\ 


Fig.  47. 


and  nitric  acids  are  formed  ;  b  b  the  leaden  chambers  containing  some 
water.  The  sulphur  is  spread  over  the  bottom  of  the  furnace,  and  the 
nitre  is  placed  in  the  cup,  shown  in  the  cut.  The  second  chamber  com- 
municates with  a  high  chimney,  for  creating  a  draught,  and  also  for 
carrying  off  the  surplus  vapors. 


Sulphuretted  Hydrogen,  or  Hydrosulphuric  Acid — HS. 

60.  Preparation.  —  Heat  sulphuret  of  antimony  in  a  retort,  with  4  or 
5  times  its  weight  of  hydrochloric  acid,  and  collect  the  gas  over  warm 
water  in  the  pneumatic  trough,  (or  by  displacement,  as  in  Art.  54.)  As 
the  bottles  are  filled  with  the  gas,  they  should  be  speedily  removed  and 
closed. 

The  action  is  represented  in  the  following  symbols  :  — 

1  eq.  sesquisulphuret  of  antimony  -f-  3  eq.  hydrochloric  acid 
=  Sb2S3  +  3HC1 
=  3HS    -f  Sb2Cl3 

=  3  eq.  sulphuretted  hydrogen  -|-  1  eq.  sesquichloride  of 
antimony. 

EXPERIMENTS. 

1.  Invert  a  jar  of  this  gas ;  apply  a  lighted  match :  the  gas  burns 
with  a  pale  blue  flame  with  the  deposition  of  sulphur. 

2.  Pour  a  few  drops  of  strong  nitric  acid  into  a  bottle  of  this  gas, 

42* 


498 


NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 


and  immediately  close  the  mouth  with  the  thumb,  protected  by  a  piece 
of  paper  :  an  explosion  takes  place  with  the  deposition  of  sulphur. 

3.  Generate  the  gas  in  a  flask  fitted  with  a  cork  and  bent  tube  pass- 
ing into  a  solution,  of  arsenious  acid,  (arsenic  of  com- 
merce :)  an  orange- colored  precipitate  is  formed  of  a 

sulphuret  of  arsenic.     Sulphuretted  hydrogen  is  much 
used  in  this  way  as  a  test  for  metals. 

4.  Transmit,  as  in  the  last  experiment,  a  current  of 
the  gas  through  liquid  ammonia  :  a  solution  of  hydro- 
sulphuret  of   ammonia  is  formed.     This  solution  is 

much  used  as  a  re- agent. 

Fig.  48. 

Phosphuretted  Hydrogen  —  PH3. 

61.  Experiment  1.  —  Put  some  thin  slices  of  phosphorus  into  a  small 
retort,  and  fill  it  completely  with  a  mixture  of  lime  and  warm  water ; 
insert  the  beak  of  the  retort  in  a  vessel  containing  warm  water ;  boil  the 


Fig.  49. 

mixture :  bubbles  of  phosphuretted  hydrogen  gas  are  given  off,  whrch, 
escaping  into  the  air,  ignite  spontaneously,  and  form  beautiful  rings  of 
smoke. 

2.  Invert  a  test  tube  filled  with  water  over  the  beak  of  the  retort  in 
which  the  gas  is  being  formed :  the  tube  is  soon  filled  with  the  gas. 
Observe  that  the  gas  is  colorless  and  transparent  like  common  air  ;  cover 
the  mouth  of  the  tube  with  the  fore  finger,  and  transfer  the  gas  into  a  jar 
of  oxygen  standing  on  the  shelf  of  the  pneumatic  trough ;  the  bubbles, 
as  they  rise  into  the  jar,  catch  fire,  giving  a  splendid  flash  of  light. 


EXPERIMENTAL    CHEMISTRY.  499 

CHLORINE  —  Cl. 

62.  Preparation.  —  Introduce  into  a  retort  hydrochloric  acid  and  black 
oxide  of  manganese,  so  as  to  form  a  thin  paste  ;  heat  the  mixture  with 
an  Argand  lamp :  chlorine  gas  is  speedily  given  off,  which  may  be  rec- 
ognized by  its  peculiar  color  and  suffocating  odor  ;  receive  the  gas  over 
warm  water  in  a  small  pneumatic  trough,  (or  by  displacement ;  see  Art. 
54.)     The  action  is  represented  by  the  following  symbols :  — 

2  eq.  hydrochloric  acid  +  1  eq.  oxide  of  manganese 
=  2HC1  +  MnO2 
=       Cl  +  MnCl  -f  2HO 
=  1  eq.  chlorine  -f  chloride  of  manganese  and  2  eq.  water. 

EXPERIMENTS. 

1.  Repeat  Exp.  1,  Art.  22. 

2.  Let  fall  powdered  antimony  into  a  bottle  of  this  gas  ;  the  metal 
ignites  spontaneously,  and  forms  a  beautiful  shower  of  flame.     Various 
other  metals  ignite  spontaneously  in  this  gas,  and  form  chlorides. 

3.  Put  a  piece  of  red  calico  moistened  with  water  into  a  bottle  of 
chlorine :  the  color  is  speedily  discharged. 

Hydrochloric  Acijf, —  HC1. 

63.  To  obtain  this  acid  in  the  gaseous  state,  introduce  into  a  retort 
common  salt  and  as  much  sulphuric  acid  as  will  form  a  thin 

paste ;  apply  the  flame  of  an  Argand  lamp,  and  receive  the  gas 
by  displacement,  as  it  is  highly  soluble  in  water.  (See  Art.  54.) 

Experiment.  —  Place  a  bottle  of  this  gas  over  a  bottle  of  am- 
moniacal  gas  :  the  gases  combine  and  form  dense  white  fumes 
of  hydrochlorate  of  ammonia. 

Liquid  hydrochloric  acid  may  be  prepared  in  considerable 
quantities  by  transmitting  a  current  of  the  gas  through  water ; 
it  may  also  be  made  on  a  small  scale,  after  the  manner  of  pre- 
paring nitric  acid,  (seepage  492,)  taking  care  in  this  case  to  put 
water  into  the  receiver  b. 

Cyanogen  —  Cy,  or  C2N. 

64.  Experiment.  —  Introduce  a  few- grains  of  cyanide  of  mercury 
(HgCy)  into  a  test  tube  fitted  with  a  cork  and  bent  tube ;  apply  the 


500          NATURAL    A^ND    EXPERIMENTAL    PHILOSOPHY. 


Fig.  51. 

flame  of  a  spirit  lamp :  cyanogen  gas  is  given  off;  ignite  the  gas  as  it 
issues  from  the  small  tube  :  it  burns  with  a  beautiful  violet  flame. 


SECTION  VI. 

COMPOSITION  OF  VEGETABLE  SUBSTANCES.  COMPOUND  OR- 
GANIC SUBSTANCES  IN  PLANTS.  FERMENTATION.  DIAS- 
TASE. GERMINATION  OF  PLANTS.  STRUCTURE  AND 
FUNCTIONS  OF  PLANTS.  FOOD  OF  PLANTS. 

COMPOSITION   OF   VEGETABLE    SUBSTANCES. 

65.  WHEN  a  piece  of  straw,  or  any  dried  vegetable  sub- 
stance, is  held  in  the  flameaof  a  candle,  the  greater  portion  is 
consumed  in  the  form  of  gases,  and  only  a  very  small  portion, 
called  the  ash,  is  left  behind.  That  portion  which  burns  away 
is  called  the  organic  part  of  the  plant,  and  that  which  remains,- 
the  ash,  is  called  the  inorganic  part.  The  organic  part  of  plants 
consists  of  four  elementary  substances,  viz.,  carbon,  oxygen,  hy- 
drogen, and  a  small  quantity  of  nitrogen.  The  inorganic  part 
consists  of  the  following  earthy  substances,  viz.,  potassa,  soda, 
lime,  silica,  magnesia,  alumina,  oxide  of  iron,  oxide  of  man- 
ganese, sulphuric  acid,  phosphoric  acid,  and  chlorine.  Al- 
though the  ash  forms  a  very  small  part  of  plants,  yet  it  seems 
to  \fe  as  essential  to  their  growth  and  existence  as  any  of  the 
elements  composing  the  organic  part.  The  proportion  in 
which  these  substances  are  found  varies  in  different  plants, 
and  even  in  different  parts  of  the  same  plant.  The  following 
tables,  by  Boussingault  and  Johnston,  give  the  composition 
of  the  organic  as  well  as  of  the  inorganic  parts  of  some  of 
our  most  valuable  plants. 


EXPERIMENTAL    CHEMISTRY. 


501 


When  all  moisture  has  been  evaporated,  100  Ibs.  of  each 
vegetable  substance  is  composed  as  follows  :  — 


Carbon. 

Oxygen. 

Hydrogen 

Nitrogen. 

Ash. 

Wheat,  

46-1 

43-4 

5-8 

2-3 

2-4 

100 

Oats  

50-7 

36-7 

6-4 

2-2 

4-0 

100 

Peas 

46-5 

40'0 

6-2 

4-2 

3-1 

100 

Potatoes,  

44-0 

44-7 

5-8 

1-5 

4-0 

100 

Wheat  Straw 

48-4 

38'9 

5-3 

0-4 

7-0 

100 

Oat  Straw,  

50-1 

39-0 

5-4 

0-4 

5-1 

100 

Pea  Straw,.  .  . 

45-8 

35-6 

5-0 

2-3 

11-3 

100 

In  100  Ibs.  of  ash  we  have  the  following  composition :  — 


• 

"Wheat. 

Oats. 

Barley. 

Wheat 
Straw. 

Oat 

Straw. 

19 

c-o 

12 

0'50 

15 

Foda,                                     

20-5 

5-0 

12 

0-75 

trace 

g 

3-0 

4'5 

7  -00 

2-75 

Magnesia,  

8 

24 

8 

1-00 

0-50 

2 

0* 

1 

2-75 

Oxide  of  Iron,  

0 

1-5 

trace 

0 

Silica,  

34 

76-5 

50 

81-00 

80-00 

Sulphuric  Acid,....?  

4 

1-5 

2-5 

1-00 

1-50 

Phosphoric  Acid                 

3-5 

3-0 

9 

5-00 

0-25 

1 

0-5 

1 

1-00 

trace 

100 

100 

100 

100 

100 

06.  Hence  it  appears  that  different  kinds  of  plants  must  ex- 
haust the  soil  of  different  proportions  of  inorganic  matter ;  thus, 
for  example,  100  Ibs.  of  the  ash  of  wheat  carry  off  19  Ibs.  of  po- 
tassa  and  34  Ibs.  of  silica,  while  that  of  barley  only  12  Ibs.  of 
potassa  and  as  much  as  50  Ibs.  of  silica.  Thus  it  is  that  some 
land  will  suit  one  kind  of  vegetables  and  not  another  kind; 
and  hence  it  is  that  two  successive  crops  of  different  kinds  of 
plants  may  grow  on  land,  when  two  successive  crops  of  the 
same  kind  would  exhaust  the  soil  of  some  of  its  most  essential 
constituents.  It  has,  however,  been  found,  that  when  any  one 
of  the  alkalies  is  absent  from  the  soil,  its  place  may  be,  to  a 
certain  extent,  supplied  by  another  alkali  without  injury  to 
the  vegetation :  thus,  when  a  soil  is  deficient  in  potassa  and 


502  NATURAL    AND    EXPERIMENTAL    PHILOSOrilY. 

soda,  then  lime  (an  alkaline  earth)  will  in  some  measure  sup- 
ply their  place  in  the  ash  of  the  plant. 

67.  As  plants  carry  off,  year  after  year,  certain  portions  of 
organic  as  well  as  inorganic  substances  from  the  land  in  which 
they  grow,  it  becomes  necessary,  in  most  soils,  that  these  sub- 
stances should  be  restored  to  the  land  in  the  form  of  manures. 

COMPOUND    ORGANIC    SUBSTANCES    IN    PLANTS. 

G8.  In  the  organic  part  of  plants,  the  four  elements  of 
which  it  is  composed  are  found  in  the  plant  in  the  form  t>f  dis- 
tinct compounds  ;  the  most  abundant  of  these  are  lignine  or 
woody  fibre,  starch,  gum,  sugar,  gluten,  and  albumen.  The 
first  four  substances  are  composed  of  carbon  and  water  only, 
and  the  last  two  substances  contain  nitrogen,  in  addition  to 
carbon,  oxygen,  and  hydrogen. 

The  composition  of  the  first  four  substances  is  as  follows  :  — 

Composition  may  be  represented. 

Lignine       -     C12 IT8   O8  12  cq.  carbon  and   8  eq.  water. 

Starch         -     C^  H10  O10  12  cq.  carbon  and  10  eq.  water. 

Gum  -     C12  Hu  On  12  cq.  carbon  and  11  eq.  water. 

Grape  Sugar    C12  H12  O12  12  eq.  carbon  and  12  eq.  water. 

The  only  difference  in  the  composition  of  these  compounds 
is,  that  they  contain  different  proportions  of  the  elements  of 
water. 

Most  of  vegetable  compounds  are  characterized  by  the  fol- 
lowing circumstances:  1.  By  being  composed  of  the  same 
elements ;  2.  By  the  facility  with  which  they  undergo  decom- 
position ;  3.  By  the  facility  with  which  many  of  them  are 
converted  into  each  other,  especially  when  a  substance  contain- 
ing nitrogen  is  present ;  4.  By  the  impracticability  of  form- 
ing them  by  the  direct  union  of  their  elements. 

These  distinct  compounds,  which  exist  ready  formed  in  the  * 
vegetable,  are  called  proximate  principles;  thus  sugar  and 
gum  are  proximate  vegetable  principles. 

Experiment.  —  Put  some  wheat  flour  in  a  fine  muslin  bag,  and  knead 
or  work  it  with  your  fingers,  while  a  small  stream  of  water  is  pOured 


EXPERIMENTAL    CHEMISTRY.  503 

upon  it ;  continue  the  process  until  the  water  ceases  to  be  milky  :  the 
substance  remaining  in  the  bag  is  a  gray  adhesive  matter  like  bird  lime, 
called  gluten ;  allow  the  milky  portion  which  has  been  washed  from 
the  bag  to  subside ;  decant  the  clear  liquid  :  the  white  deposition  is 
called  starch ;  take  the  clear  liquid  and  boil  it :  white  flakes  of  albumen 
are  formed,  a  substance  very  similar  in  its  nature  to  the  white  of  an  egg. 
Gum  and  sugar  are  dissolved  in  the  water. 

Perform  the  same  process  with  grated  potato :  in  this  case,  fibrous 
matter  is  left  in  the  bag,  the  other  portions  being  the  same  as  in  the 
preceding  experiment. 

69.  Lignine,  starch,  gum,  and  sugar,  being  so  similar  in 
composition,  may  readily  be  converted  into  each  other.  Thus, 
for  example,  starch  may  be  converted  into  gum  by  roasting  at 
a  temperature  above  that  of  boiling  water ;  lignine  may  be 
converted  into  gum  by  the  action  of  strong  sulphuric  acid ; 
and  the  gum  thus  formed  maybe  converted  into  sugar  by  add- 
ing water,  and  boiling  the  mixture  for  some  hours ;  and  so  on 
to  other  cases  of  transformation. 


EXPERIMENTS. 

1.  Dissolve  some  starch  in  boiling  water  :  a  thick  jelly  is  formed, 
which,  after  being  dried,  has  the  appearance  of*  glue ;  this  jelly  is  insol- 
uble in  cold  water,  and  is  rendered  blue  by  the  addition  of  a  solution  of 
iodine.     (See  Exp.  2,  Art.  21.)     To  the  thick  solution  of  starch  add  an 
infusion  of  vegetated  barley  of  the  malting :    the  starch  grows  more 
liquid,  and  in  a  short  time  its  consistence  entirely  disappears ;  evaporate 
to  dryness,  and  a  yellow  jelly-like  mass  is  obtained,  which  is  now  readily 
dissolved  by  cold  water,  whereas  starch  is  insoluble  in  cold  water.     To  a 
solution  of  this  substance  add  a  solution  of  iodine :  a  red  wine  color  is 
produced.     This  yellow  substance  is  a  gum  called  dextrine ;  it  is  used  in 
the  place  of  gum  arabic  for  stiffening  calico.     There  is  evidently  some 
active  agent  in  the  vegetating  barl%-,  which  has  produced  these  changes 
in  the  starch  :  this  agent  has  been  called  diastase. 

2.  Boil  some   diluted  sulphuric   acid   (1  part  of  acid  to    12    parts 
of  water)  in  a  porcelain  dish ;  add  gradually  some  starch  paste  :  the 
starch  is  dissolved  ;  test  a  portion  of  the  solution  by  me^ns  of  a  solution 
of  iodine :  a  red  wine  color  is  produced,  as  in  the  last  experiment ;  con- 
tinue the  boiling  for  a  short  time  longer,  and  the  iodine  will  cease  to 
produce  any  change  of  color.     Take  the  liquid  in  the  evaporating  dish, 
and  add  to  it  powdered  chalk  until  the  acid  is  neutralized,  and  allow 


504  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

the  sulphate  of  lime  to  subside  :  the  clear  liquid  is  sweet,  and  crystals 
of  sugar  may  be  obtained  by  evaporating  a  portion  of  the  water  by  a 
slow  heat.  In  this  process  no  gas  is  given  off,  and  the  acid  suffers  no 
change.  The  only  difference  in  the  composition  of  starch  and  sugar  is, 
that  the  latter  contains  more  of  the  elements  of  water  than  the  former. 

Berzelius  designates  this  peculiar  action  exerted  by  the  sul- 
phuric acid  in  converting  starch  into  sugar  the  catalytic  force, 
or  the  force  of  catalysis. 

70.  Fermentation.  —  This  term  is  used  generally  to  express 
those  changes  that  are  spontaneously  effected  in  organic  sub- 
stances by  the  reaction  of  their  elements.     Thus,  when  a  so- 
lution of  grape  sugar,  to  which  ferment  or  common  yeast  has 
been  added,  is  kept  for  some  time  at  a  moderate  heat,  the  mix- 
ture froths  up,  in  consequence  of  the  escape  of  carbonic  acid 
gas,  the  sweet  taste  of  the  solution  gradually  disappears,  and 
when  the  fermentation  has  ceased,  spirit,  or  alcohol,  is  found 
in  the  water.     This  spirit  is  given  off  in  a  concentrated  form 
by  evaporation  at  a  temperature  below  that  of  boiling  water. 
Alcohol  (C4H6O2)  contains  less  oxygen  and  carbon  than  sugar ; 
hence  the  escape  of  carbonic  acid  in  order  to  change  the  sugar 
into  alcohol.     Thus  sugar,  or  CI2H12OI2,  becomes  2C4H6O2,  or 
2  eq.  alcohol  and  4CO?,  or  4  eq.  carbonic  acid.     Yeast,  as 
well  as  all  substances  which  have  the  property  of  inducing  or 
exciting  fermentation,  contains  nitrogen,  in  addition  to  carbon, 
oxygen,  and  hydrogen. 

71.  When  a  mixture  of  diluted  spirit  and  yeast  is  exposed 
to  the  air,  oxygen  is  absorbed,  and  acetic  acid  or  vinegar  is 
formed.     The  composition  of  dry  acetic  acid  is  C4H3O3 ;  that 
is,  it  may  be  represented  by  4  eq.   carbon  and  3   eq.  water. 
Hence  the  action  may  be  represented  as  follows :  — 

1  eq.  alcohol  +  4  eq.  oxygen  =  C4H6O2  -f-  O4 

=  C4H3O3  -f  3HO, 

or  1  eq.  acetic  acid  -f-  3  eq.  water. 

* 

In  both  of  these  cases  of  fermentation  the  yeast  merely  acts 
as  a  stimulating  agent. 

72.  Besides   the   proximate   vegetable   principles   already 


EXPERIMENTAL    CHEMISTRY.  505 

enumerated,  there  are  several  vegetable  acids,  oils,  fatty  mat- 
ters, and  the  peculiar  substance  called  diastase,  which  pro- 
duces an  important  action  in  relation  to  the  growth  of  plants. 
Vegetable  acids.  —  The  most  common  vegetable  acids  are, 
acetic  acid  (vinegar),  malic  acid  (the  acid  of  apples),  oxalic 
acid  (from  common  sorrel),  tartaric  acid  (in  grapes),  citric 
acid  (the  acid  of  lemons). 

GERMINATION.      DIASTASE. 

73.  When  a  seed  is  planted  it  begins  to  sprout ;  that  is,  it 
shoots  a  sprout  upwards  into  the  air,  and  sends  a  root  down- 
wards into  the  soil.     At  this  stage  of  the  life  of  the  young 
plant  it  must  live  upon  the  starch  and  gluten  contained  in  the 
seed  alone.     In  order  to  render  these  substances  soluble  in 
water,  and  thereby  available  for  the  food  of  the  plant,  there  is 
formed  out  of  the  gluten,  at  the  base  of  the  germ,  the  peculiar 
substance  called  diastase.     This  substance  renders  the  starch 
soluble  in  the  sap,  and  it  is  thus  conveyed  to  the  shoot  and 
root  of  the  young  plant.     The  starch  in  this  state  of  solution 
becomes  sugar.     As  the  plant  advances  in  its  growth  it  begins 
to  have  leaves,  and  at  this  stage  the  sugar  is  changed  into 
woody  fibre,  which  forms  the  stem.     By  the  time  the  starch 
and  gluten  are  exhausted  from  the  seed,  the  plant  has  acquired 
all  the  functions  necessary  for  taking  up  food  from  the  air  and 
the  soil.     A  similar  process  takes  place  in  the  formation  of 
malt,  where  the  germination  of  the  barley  is  stopped  when 
the  sugar  is  formed. 

STRUCTURE  AND  FUNCTIONS  OF  PLANTS.   FOOD  OF  PLANTS. 

# 

74.  A  complete  plant  has  three  parts  which  are  essential  to 
its  growth  :  a  root,  which  throws  out  fibres  into  the  soil ;  a 
trunk    or  stem,  which  rises  into  the  air;  and  leaves,  which 
present  an  extended  surface  to  the  action  of  the  air.     Each 
of  these  three  parts  performs  peculiar  functions  or  offices  in 
the  growth  of  tho  plant. 

1.  The  trunk,  or  stem,  consists  of  three  parts  :  in  the  centre 
43 


506  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

is  the  pith7  next  the  pith  is  the  wood,  and  the  bark  encloses 
the  whole. 

The  pith  consists  of  very  small  horizontal  tubes ;  the  wood 
and  inner  bark  are  made  up  of  longitudinal  tubes  connected 
together  for  conveying  the  sap  between  the  roots  and  leaves  ; 
the  vessels  in  the  wood  convey  the  sap  from  the  roots  to  the 
leaves,  and  the  vessels  in  the  inner  bark  convey  the  sap  from 
the  leaves  to  the  roots  ;  thus  in  a  growing  plant  there  are  cur- 
rents of  sap  continually  ascending  and  descending.* 

2.  The  root  on  leaving  the  stem  has  the  same  structure  as 
the  trunk ;  but  the  finely-extended   tendrils   consist   of    one 
white,  uniform,   spongy  mass,  for  the  purpose  of    absorbing 
liquid  food  from  the  soil. 

3.  The  leaf  consists  of  fibres,  which  are  continuations  of 
the  wood,  together  with  the  green  portion,  which  is  a  continu- 
ation of  the  bark.     The  under  part  of  the  leaf  is  full  of  pores, 
which  communicate  with  the  hollow  tubes  of  the  inner  bark. 
It  has  already  been  explained  (Exp.  6,  Art.  15,  and  Art.  18) 
that  in  the  daytime  the  leaves  are  continually  absorbing  car- 
bonic acid  gas  from  the  air,  and  throwing  off  oxygen  ;  thus 
carbonic  acid  is  decomposed  by  the  plant  —  the  carbon   is 
retained  as  food,  while  the  oxygen  is  rejected.     The  reverse 
of  this  process  is  going  on  at  night,  but  so  slowly  as  scarcely 
to  interfere  with  the  general  effect.     Carbonic  acid  also  enters 
the  plant  through  the  roots.     Some  suppose  that  carbon  enters 
the  plant  by  the  roots  in  the  form  of  ulmic  acid,  a  substance 
composed  of  carbon  and  water  only. 

75.  The  elements  composing  the  organic  part  of  plants  are 
always  absorbed  in  a  state  of  combination,  and  the  substances 
forming  the  inorganic  part  must  be  in  a  state  of  solution,  in 
order  to  be  sucked  in  by  the  roots.  The  food  of  plants  must 
contain  the  various  elements  which  enter  into  their  composi- 
tion. In  general,  the  substances  which  afford  this  food  are 

*  The  ascent  of  the  sap  probably,  in  some  measure,  depends  on  the  prin- 
ciple of  endosmose  and  exosmose.  (See  Art.  39,  p.  89,  of  the  Treatise  on  Hy- 
drostatics.) 


EXPERIMENTAL    CHEMISTRY.  507 

carbonic  acid,  water,  and  ammonia,  derived  from  the  air  as 
well  as  the  soil ;  and  certain  saline  and  earthy  substances,  de- 
rived exclusively  from  the  soil.  Light  and  heat  (and  proba- 
bly electricity)  stimulate  the  functions  of  plants,  and  are  abso- 
lutely necessary  to  their  growth  and  full  development.  Light 
is  also  essential  to  the  formation  of  the  coloring  matter  in 
plants. 

It  will  now  be  easy  to  see  how  the  plant  should  form  woody 
fibre,  starch,  sugar,  gum,  or  vinegar,  all  of  which  substances 
consist  of  carbon  and  water  only,  united  in  different  propor- 
tions. Ammonia  and  nitric  acid  supply  the  plant  with  nitro- 
gen. 

SECTION  Vn. 

SOILS.  THEIR  COMPOSITION.  ORGANIC  AND  INORGANIC 
PARTS.  SALINE  AND  EARTHY  PARTS.  PHYSICAL  CHAR- 
ACTER OF  SOILS.  TO  SEPARATE  THE  SAND  FROM  THE 
CLAY.  TO  DETERMINE  THE  QUANTITY  OF  LIME,  OF  OR- 
GANIC MATTER,  AND  OF  SALINE  MATTER,  IN  A  SOIL. 
ORIGIN  OF  SOILS.  MECHANICAL  PROPERTIES  OF  SOILS. 
CHEMICAL  PROPERTIES  OF  SOILS. 

COMPOSITION    OF   SOILS. 

76.  Soils,  like  plants,  are  composed  of  organic  as  well  as 
of  inorganic  matter. 

The  organic  part  of  soils  is  chiefly  derived  from  the  re- 
mains of  vegetable  and  animal  substances.  Peaty  soils  con- 
tain a  large  proportion  of  organic  matter,  while  good  wheat 
lands  contain  only  about  one  twentieth  of  their  whole  weight. 
This  organic  matter  in  the  soil  has  been  called  humus,  which, 
by  the  action  of  alkaline  substances,  is  resolved  into  ulmic  and 
humic  acids.  As  the  vegetable  matter  undergoes  decay,  this 
organic  portion  of  the  soil  also  gives  to  the  land  the  various 
inorganic  substances  found  in  its  ash. 

77.  The  inorganic  part  of  soils  consists  of  certain  saline 


508          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

soluble   substances,   and    of    certain    earthy   insoluble    sub- 
stances. 

1.  The  saline  soluble  substances  consist,  in  general,  of  com- 
mon salt,  sulphates  of  soda  and  magnesia,  gypsum,  with  small 
portions  of  the  nitrates  of  potassa,  soda,  and  lime,  and  of  the 
chlorides  of  calcium,  magnesium,  and  potassium,  together  with 
ammoniacal  salts.     From  these  soluble  compounds  the  plant 
obtains  nearly  all  the  saline  matter  contained  in  its  ash.     The 
rain  dissolves  these  saline  substances,  and  carries  them  into 
the  subsoil ;  but  in  dry,  warm  weather,  they  reascend  to  the 
surface,  and  are  thus  brought  in  contact  with  the  roots  of  the 
growing  plant.    Thus  fine  warm  weather  accelerates  the  ripen- 
ing of  corn  and  other  valuable  grain. 

2.  The  earthy  insoluble  substances  in  the  soils  never  consti- 
tute less  than  nine  tenths  of  their  whole  weight.     The  prin- 
cipal ingredients  of  this  earthy  matter  are  silica,  in  the  form 
of  sand,  alumina,  mixed  with  sand,  in  the  form  of  clay,  and 
carbonate  of  lime.     "Where  the  soil  has  a  red  color,  the  oxide 
of  iron  is  generally  present.     Minute  traces  of  phosphate  of 
lime  may  also  be  detected  in  most  good  soils. 

PHYSICAL    CHARACTER    OF    SOILS. 

78.  The  relative  proportions  of  sand,  clay,  and  lime  in  a 
soil  give  it  a  peculiar  physical  character.     When  a  soil  con- 
tains only  a  small  proportion  of  clay,  it  is  called  a  sandy  soil ; 
when  the  quantities  of  sand  and  clay  are  nearly  equal,  it  is 
called  a  loamy  soil,  or  clay  loam,  according  as  the  quantity  of 
sand  is  greater  or  less  than  the  clay ;  when  the  clay  is  much 
in  excess,  it  is  called  clay  loam,  or  strong  clay,  as  the  case  may 
be.     Good  arable  land  rarely  contains  more  than  one  third 
part  of  its  weight  of  clay. 

79.  To  separate  the  sand  from  the  clay  in  a  soil.  —  Take  about  half  an 
ounce  of  soil,  and  boil  it  in  about  half  a  pint  of  water,  in  a  porcelain 
dish,  until  it  is  completely  diffused  through  the  water ;  after  shaking, 
let  the  mixture  stand  for  a  minute,  to  allow  the  sand  to  settle  to  the 
bottom  of  the  vessel,  while  the  clay  remains  suspended  in  the  fluid ; 


EXPERIMENTAL    CHEMISTRY.  509 

pour  off  the  water  with  the  floating  clay  into  another  vessel,  and  allow 
the  clay  now  to  settle.  The  sandy  portion  of  the  soil  will  be  found  in 
the  first  vessel,  and  the  clayey  portion  at  the  bottom  of  the  second.  The 
sand  and  clay  may  now  be  dried  and  weighed  separately,  and  the  rela- 
tive weights  will  give  the  proportion  in  which  they  subsist  in  the  soil. 

80.  If  a  soil  contains  more  than  one  twentieth  of  its  weight 
of  carbonate  of  lime,  it  is  called  marl ;  and  if  more  than  one 
fifth,  it  is  called  calcareous  soil. 

To  determine  the  quantity  of  lime  in  a  soil.  —  Take  100  grains  of  the 
soil,  (which  has  been  previously  heated  to  redness,  to  destroy  the  vegeta- 
ble matter,)  and  diffuse  it  through  about  half  a  pint  of  distilled  water ; 
add  about  an  ounce  of  hydrochloric  acid,  and  allow  the  mixture  to 
stand  for  a  few  hours,  observing  to  stir  it  from  time  to  time.  Bubbles  of 
carbonic  acid  are  given  off.  After  the  action  has  ceased,  pour  off  the 
clear  liquid ;  dry  and  then  heat  the  residue  to  redness,  and  weigh  it : 
the  loss  is  nearly  the  weight  of  lime  and  carbonate  of  lime  in  the  soil. 

81.  To   determine  the  quantity  of  organic  matter.  —  Dry  about  an 
ounce  of  the  soil  on  paper  in  an  oven,  at  a  heat  which  does  not  char  the 
paper ;  burn  about  200   grains  of  this  dry  soil :  the  loss  is  nearly  the 
weight  of  the  organic  matter  contained  in  it. 

82.  To  determine  the  quantity  of  saline  matter.  —  Take  2  Ibs.  of  dry 
soil,  and  boil  it  in  about  a  quart  of  distilled  water  ;  after  allowing  the 
solid  matter  to  subside,  pour  off  the  clear  liquid,  and  evaporate  to  dry- 
ness  at  a  moderate  heat ;  weigh  the  residue,  and  it  will  give  the  quan- 
tity of  soluble  saline  matter  in  the  soil.     In  a  good  soil   this  saline 
matter  may  weigh,  upon  an  average,  about  20  grains. 

ORIGIN    OF    SOILS. 

83.  Soils  owe  their  origin  to  the  Disintegration  or  gradual 
crumbling  down  of  rocks,  by  the  action  of  water,  frost,  air, 
and  various  chemical  agents.     Hence  soils,  in  general,  derive 
their  peculiar  character  from  the  geological  strata  upon  which 
they  lie,  or  from  the  nature  of  the  rocks  in  the  adjacent  hills 
or  mountains. 

MECHANICAL    PROPERTIES    OF    SOILS. 

84.  Sandy  and  marly  soils  are  heavy,  while  peaty  soils  are 
light.     Strong  clays  and  peaty  soils  absorb  and  retain  moist- 
ure ;  hence  they  are  damp  and  cold ;  hence,  especially,  the 

43* 


510          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

necessity  for  draining  such  soils.  Sandy  soils  neither  absorb 
nor  retain  much  moisture  ;  hence  such  soils  become  scorched 
with  the  heat  of  summer,  and  the  plants  growing  upon  them 
are  burned  up.  In  rainy  seasons,  however,  sandy  soils  fre- 
quently sustain  a  luxuriant  vegetation,  while  the  plants  upon 
a  clayey  land  almost  perish  from  the  excess  of  moisture. 

85.  Heat  causes  clay  and  peat  to  contract;  in  doing  so,  the 
soil  compresses  the  roots  of  the  plants,  and  prevents  the  access 
of  air,  and  thus  the  growth  of  the  plant  is  retarded. 

86.  The  absorbent  power  of  clay  is  useful  in  a  soil,  for  dur- 
ing the  hot  and  dry  season  of  the  year,  in  the  cool  period  of 
the  night,  the  clay  absorbs  the  dew  that  falls  upon  it,  and  re- 
tains the  moisture  with  great  tenacity. 

87.  In  order  that  plants  may  come  to  perfection,  it  is  neces- 
sary that  the  soil  on  which  they  grow  should  attain  a  certain 
degree  of  warmth.     Damp  lands  are  cold,  for  the  continual 
evaporation  of  the  moisture  carries  off  the  heat  of  the  sun  ; 
hence  the  necessity  of  drainage. 

88.  These  observations  show  the  value  of  a  due  admixture 
of  clay  and  sand  in  order  to  form  a  mixture  having  all  the 
mechanical  qualities  of  a  fertile  soil,  where  the  earthy  constit- 
uents are  so  adjusted  that  "  the  loose  and  porous  qualities  of 
the  one  are  corrected  by  the  plastic  and  retentive  qualities  of 
the  other."     It  is  a  remarkable  fact,  that  a  mixture  of  alumi- 
na, silica,  and  lime  absorbs  gaseous  matter  as  well  as  moisture, 
better  than  any  of  these  earths  taken  by  itself. 

CHEMICAL    PROPERTIES    OF   SOILS. 

89.  Soils  not  only  sustain  a  plant  in  an  erect  position  and 
afford  it  food,  but  they  are  the  medium  in  which  various  chem- 
ical actions  are  gradually  and  constantly  going  on,  in  the  prep- 
aration of   different  substances  essential   to  the  growth  of 
plants.     Thus  lime  is  constantly  decomposing  vegetable  and 
animal  matter  in  the  soil,  and  thereby  preparing  food  for  the 
plant.     Thus  organic  substances  in  the  soil  aid  in  absorbing 
ammonia  and  carbonic  acid  from  the  air.     Thus  little  grains 


EXPERIMENTAL    CHEMISTRY.  511 

of  alkaline  silicates  are  gradually  reduced  to  powder,  and  in 
this  state  water  dissolves  the  alkaline  matter.  (See  Exp.  1, 
Art.  28.) 

90.  A  fertile  soil  should  not  only  coalain  all  the  elements 
essential  to  the  growth  of  a  plant,  but  they  should  exist  in  a 
due  proportion.  A  deficiency  of  one  substance,  or  an  excess 
of  another,  may  equally  contribute  to  deteriorate  the  quality 
of  the  land.  Hence  the  utility  of  artificial  applications  to 
land,  whereby  the  farmer  is  enabled  to  supply  what  may  be 
deficient,  or  in  some  degree  to  neutralize  the  influence  of  what 
may  be  in  excess.  The  following  analyses  of  three  different 
soils,  by  Dr.  Sprengel,  afford  a  striking  illustration  of  these 
remarks. 

1000  parts  of  each  soil  contained  as  follows :  — 

No.  1.  No.  2.  No.  3. 

Fine  earthy  and  organic  matter,      -     937          839  599 

Silicious  sand,         -                         -       45           160  400 

Saline  soluble  matter,                               18               1  1 

1000         1000  1000 

1000  parts  of  the  fine  earthy  and  organic  matter  contained,  — 

No.  1.  No.  2.      No.  3. 

Organic  matter,                                         97  50  40 

Silica,  -  -                        648  833            778 

Alumina,                                                    57  51  91 

Lime,    -                                                      59  18  4 

Magnesia,                                                    8-5  8  1 

Oxide  of  iron,  -                                       61  30  81 

Oxide  of  manganese,     -                            1  3  £ 

Potassa,                                        -              2  trace          trace 

Soda,    -  -  4 

Ammonia,         -  -        trace 

Chlorine,  2 

Sulphuric  acid,                                             2  f 

Phosphoric  acid,  -                             4-5  1| 

Carbonic  acid,  -                                        40  4£ 

Loss,     -                                                      14  4£ 

1000         1000  1000 


512          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

No.  1  is  a  highly  fertile  soil,  which  had  grown  corn  and 
pulse  crops  without  the  application  of  any  manure.  This  soil 
seems  to  contain  all  the  essential  constituents  of  plants.  No. 
2  is  a  fertile  soil  whieh  required  to  be  manured  with  gypsum. 
The  analysis  indicates  a  deficiency  of  soluble  saline  matter, 
with  only  traces  of  potassa,  soda,  and  sulphuric  and  other 
acids.  No.  3  is  a  barren  soil ;  it  is  deficient  in  organic  mat- 
ter; potassa,  soda,  &c.,  are  almost  wanting;  lime,  oxide  of 
iron,  and  silica  seem  to  be  largely  in  excess.  In  order  to  ren- 
der this  soil  productive,  it  would  require,  not  only  to  have 
added  those  substances  which  are  absent,  but  some  other  sub- 
stances which  would  tend  to  neutralize  the  matters  in  excess. 


SECTION  VIII. 

IMPROVEMENT  OF  SOILS.  MECHANICAL  OPERATIONS  : 
DRAINING,  PLOUGHING,  ETC.  MANURING  :  VEGETABLE, 
ANIMAL,  AND  MINERAL  MANURES.  SPECIAL  MANURES. 
ROTATION  OF  CROPS.  FALLOWING.  IRRIGATION. 

IMPROVEMENT    OF    SOILS. 

91.  Land  may  be  improved  by  working  it,  that  is,  by  me- 
chanical operations,  such  as  draining,  ploughing,  &c. ;  or  by 
improving  the  quality  of  the  soil  by  the  application  of  ma- 
nures. 

MECHANICAL    MEANS    OF   IMPROVING   LAND. 

92.  Draining.  —  It  has  already  been  shown  (Art.  87)  that 
damp  lands  are  cold  and  unproductive.     The  first  considera- 
tion, therefore,  with  the  farmer  in  reference  to  such  soils  is  to 
have  all  redundant  moisture  carried  off  by  means  of  drains. 
The  advantages  of  drainage  are  further  shown  by  the  follow- 
ing circumstances.     When  there  is  too  much  water  in  a  soil, 
the  food  of  the  plant  is  either  washed  down  to  the  subsoil,  or 
it  enters  the  roots  in  a  very  diluted  state.     When  a  soil  has 
been  drained  and  ploughed,  it  is  no  longer  close  and  adhesive, 


J&PERIMENTAL    CHEMISTRY.  513 

but  permits  the  air  to  penetrate  through  it,  and  the  roots  to 
extend  themselves  in  all  directions.  Moreover,  a  more  health- 
ful decomposition  of  the  organic  matter  goes  on  in  dry  soils 
than  in  damp  ones. 

There  are  few  soils  which  may  not  be  benefited  by  drain- 
age. It  is  especially  beneficial  to  damp  clay  and  peaty  soils. 
When  the  soil  is  a  clay  with  sand  or  gravel  for  the  subsoil,  it 
will  be  sufficient  if  the  surface  is  drained  ;  but  when  the  soil 
is  sandy,  with  clay  for  the  subsoil,  the  drain  should  go  down 
into  the  subsoil  ;  otherwise  the  land  will  be  damp  and  cold. 
To  prevent  the  soil  being  washed  away,  the  fall  of  drains 
should  be  gentle.  Land  should  always  be  drained  some  time 
before  ploughing.  Drain  pipes  made  of  porous  burnt  clay, 
fitting  into  each  other,  are  now  generally  adopted  for  agricul- 
tural purposes. 

93.  Ploughing  in  general,  especially  combined  with  drain- 
age, allows  water,  air,  and  other  gases  to  come  in  contact  with 
the  roots  of  the  plants,  destroys  unhealthful  acidity  in  the  soil, 
and  promotes  the  decomposition  of  vegetable  matter. 

94.  Subsoil  and  deep  ploughing  especially  bring  new  min- 
eral manure,  such  as  lime,  to  the  surface.     Agriculturists  con- 
sider that  the  subsoil  plough  should  not  be  used  until  after  the 
land  has  been  drained  for  one  year.     The  reason  of  this  must 
be  obvious  ;  damp  soils  are  merely  cut  by  the  plough,  whereas 
dry  soils  are  broken  to  powder  when  a  heavy  plough  passes 
through  them. 

MANURING   AS   A   MEANS    OP   IMPROVING   SOILS. 

95.  Manures  are  divided  into  three  classes,  viz.,  vegetable 
manures,  animal  manures,  and  mineral  manures. 

VEGETABLE    MANURES. 

96.  These  manures  serve  to  open  the  pores  of  the  land,  and 
to  supply  organic  as  well  as  inorganic  food  to  plants.     Vege- 
table matter  may  be  used  as  a  manure  either  in  the  green 
state  or  in  the  dry  state. 


514          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

Green  manures.  —  When  green  vegetable  substances  are  put 
into  the  soil,  they  undergo  a  rapid  decay,  yielding  a  speedy 
supply  of  food  to  the  growing  plant ;  on  the  contrary,  dry  ma- 
nures decay  more  slowly,  but  act  more  permanently  upon  the 
land.  The  cleanings  of  ditches,  hedge  sides,  &c.,  turnip  and 
potato  tops,  mixed  with  earth,  and  formed  into  a  compost  heap, 
constitute  an  enriching  application  to  the  soil.  In  some  parts 
of  this  country  turnip  seed  is  sown  at  the  close  of  harvest, 
and  at  the  end  of  two  months  the  green  crop  is  ploughed  into 
the  land.  Sea  weeds  form  a  valuable  green  manure. 

97.  Dry   manures.  —  Dry  vegetable   substances,   such    as 
straw,  sawdust,  &e.,  decay  very  slowly ;  it  is  desirable,  there- 
fore, before  applying  such  substances  to  the  land,  that  they 
should  be  mixed  with  some  matter  which  tends  to  promote 
fermentation.     Sawdust  mixed  with  soil  and  common  weeds, 
laid  up  in  a  compost  heap,  and  from  time  to  time  watered  with 
the  liquid  manure  of  the  farm-yard,  is  converted  into  a  valu- 
able vegetable  mould.     If  the   fermentation    be   not  carried 
beyond  a  certain  point,  this  compost  exercises  a  gradual  and 
prolonged  action  on  the  growing  plants :  on  the  contrary,  if  it 
be  laid  on  the  land  when  in  a  complete  state  of  fermentation, 
the  action  is  immediate ;  hence  the  application  of  the  latter 
kind  of  manures  to  turnips  and  other  crops  which  require  to 
be  brought  into  a  condition  of  rapid  growth.      Charcoal  pow- 
der, malt  dust,  bran,  rape  dust,  soot,  tanner's  bark,  &c.,  are  the 
most  common  dry  manures  in  use. 

ANIMAL    MANURES. 

98.  Animal  manures  are  the  most  energetic  in  their  action, 
in  consequence  of  the  nitrogen  they  contain,  which  exists  in 
them  in  the  form  of  ammoniacal  salts  :  these  salts  are  amongst 
the  most  powerful  agents  in  promoting  vegetation.     The  value 
of  guano  as  an  application  to  the  soil  depends  chiefly  on  the 
quantity  of  ammonia  which  it  contains.     According  to  Liebig, 
the  air  immediately  in  contact  with  the  soil  contains  small  por- 
tions of  ammonia,  which  is  being  continually  absorbed  by  the 


EXPERIMENTAL    CHEMISTRY.  515 

soil.  The  soluble  portion  of  manures  is  most  valuable,  in 
consequence  of  the  volatile  substances  which  it  contains ;  and 
hence  the  intelligence  and  industry  of  a  farmer  are  shown  by 
the  care  he  takes  of  his  barn  yard.  In  warm  weather  the 
mixed  manure  heap,  or  compost  heap,  should  be  watered,  and 
a  free  current  of  air  allowed  to  pass  over  it,  in  order  to  check, 
in  some  degree,  the  process  of  fermentation,  which  causes  the 
carbonate  of  ammonia  to  escape  into  the  air.  In  order  still 
further  to  secure  the  volatile  matters,  the  heap  should  be  cov- 
ered over  ^tli  a  layer  of  soil,  or,  in  other  cases,  with  the  sul- 
phate of  lime  ;  these  earths  absorb  and  fix  the  vapors,  and  are 
thus  converted  into  valuable  applications  to  land.  Quicklime 
should  never  be  put  into  the  compost  heap,  for  it  decomposes 
the  salts  of  ammonia,  and  thus  the  most  valuable  portion  of 
the  manure  would  be  dissipated  into  the  atmosphere.  As 
there  is  always  a  loss  during  fermentation,  the  judgment  of 
the  farmer  must  be  exercised  as  to  the  proper  time  for  laying 
the  fermenting  manure  upon  his  land  :  this  time  must,  in  some 
degree,  depend  upon  the  nature  of  the  soil  and  the  crops  to 
be  reared.  To  cold  soils,  for  example,  fully  fermented  manure 
is  most  valuable,  as  it  tends  to  warm  the  soil,  and  to  stimulate 
the  growth  of  the  plant. 

99.  Boussingault  gives  the  following  analysis  of  an  average 
farm  yard  manure  :  — 

In  100  parts  of  the  manure  we  have 

Carbon,  7-41 

Oxygen,  J            5.34 

Hydrogen,       -  -             0-87 

Nitrogen,         -  0-41 

Salts  and  earthy  substances,  -                                       6-67 

Water,  79-30 

100-00 
MINERAL    MANURES. 

100.  Lime,  shell  sand,  and  marl.  —  Lime  is  the  most  impor- 
tant of  all   the  mineral  applications  to  land.     It  serves  a  me- 
chanical purpose  by  giving  a  proper  consistency  to  soils,  and 


516         NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

it  acts  chemically  by  decomposing  various  organic  substances, 
at  the  same  time  absorbing  and  fixing  their  gaseous  products, 
and  rendering  vegetable  as  well  as  mineral  substances  soluble 
which  were  not  so  before.*  Mr.  Moffat,  in  an  able  paper, 
(published  in  the  "  Journal  of  the  Northumberland  Agricultu- 
ral Society"  for  the  year  1849,)  adduces  the  following  exper- 
iment to  illustrate  the  mode  in  which  lime  acts  on  the  soil :  — 

Experiment. —  "Take  some  sawdust,  or  any  fibrous  matter,  and  boil 
it  in  water,  so  as  to  extract  all  its  soluble  matter ;  wash  it  well  with  cold 
water,  and  strain,  so  as  to  leave  it  only  in  a  moist  state  ;  tljpn  add  to  it 
one  fifth  part  of  caustic  lime,  and  close  the  mixture  up  in  a  bottle  for 
two  or  three  months.  After  this  period  you  will  find  the  lime  to  have 
assumed  a  broAvnish  color,  effervescent  when  vinegar  is  poured  upon  it, 
which  indicates  the  presence  of  carbonic  acid ;  and  when  water  is  again 
boiled  with  the  mass,  it  will  gain  a  fawn  color,  and  by  evaporation  leave 
a  fawn- colored  powder,  consisting  of  nine  combined  with  vegetable  ex- 
tract. The  sawdust,  previous  to  the  action  of  the  lime,  was  perfectly 
insoluble  hi  water  ;  it  is  now  converted  into  a  brownish  powder,  which 
dissolves  in  large  quantity  in  water.  Now,  this  is  precisely  an  example 
of  the  change  produced  by  the  action  of  lime  in  a  caustic  state  upon  the 
insoluble  fibrous  matters  of  the  soil."  Mr.  Moffat  further  observes, 
"  Caustic  lime  decomposes  all  the  salts  and  combinations  of  ammonia, 
combining  with  their  acids  by  reason  of  its  stronger  alkaline  affinity, 
and  dissipating  the  ammonia  into  the  atmosphere ;  hence  lime  should 
never  be  applied  with  guano,  nor  farm  yard  manure,  as  a  great  portion 
of  the  nutritive  quality  of  these  manures  resides  in  the  salts  of  ammonia 
they  contain." 

When  vegetable  matter  abounds  in  a  soil,  a  considerable 
portion  of  lime  may  be  used,  to  promote  the  decomposition. 
Stiff  clay  lands,  after  draining,  should  be  well  limed ;  on  the 
contrary,  light  lands,  where  there  is  neither  much  moisture 
nor  vegetable  matter,  do  not  require  such  a  quantity.  Striking 
effects  are  produced  by  a  due  application  of  lime  to  pasture 
and  arable  lands. 

*  Insoluble  compounds  of  silica  and  potassa  exist  in  many  of  our  rocks  : 
now,  \\hen  these  earths  are  crushed  and  mixed  with  lime  and  water,  it  has 
been  found  that,  after  a  certain  time,  the  silica  and  potassa  are  converted 
into  a  soluble  form.  No  doubt  these  changes  take  place,  to  a  limited  extent, 
in  the  soil. 


EXPERIMENTAL    CHEMISTRY.  517 

The  effects  of  lime  gradually  disappear,  and  after  a  few 
years  the  land  returns  to  its  original  state,  unless  fresh  lime  be 
added. 

Lime  is  removed  from  the  soil,  —  first,  by  sinking  through 
the  loose  soil ;  secondly,  by  rains  which  wash  it  away ;  and 
thirdly,  by  the  crops  carrying  off  certain  portions  of  lime  in 
the  form  of  the  carbonate. 

Marl  and  shell  sand,  besides  other  fertilizing  matters,  con- 
tain a  large  quantity  of  carbonate  of  lime ;  their  action  upon 
land  is  similar  to  that  of  mild  lime.  Sulphuret  of  iron  (iron 
pyrites)  is  found  in  some  soils.  This  insoluble  substance  has 
no  chemical  action  ;  but  when  it  has  been  for  a  length  of  time 
exposed  to  the  action  of  the  air,  it  absorbs  oxygen,  and  is  con- 
verted into  sulphate  of  iron,  (green  vitriol,)  which  is  highly 
soluble,  and  injurious  to  plants.  Now,  the  addition  of  carbo- 
nate of  lime  decomposes  this  salt,  forming  sulphate  of  lime  and 
the  inert  oxide  of  iron,  with  the  escape  of  carbonic  acid  gas. 

Sulphate  of  lime  may  be  used  with  advantage  for  all  kinds 
of  crops  ;  but  it  is  especially  applicable  to  clover,  pea,  and  bean 
crops.  The  sulphates  generally  supply  sulphur  to  plants. 

Sulphate  of  magnesia,  as  a  top  dressing,  has  been  applied 
with  great  benefit  to  young  wheat. 

Sulphate  of  soda  (Glauber  salts)  has  been  beneficially  used 
for  turnip  crops  ;  and,  mixed  with  nitrate  of  soda,  it  has  given 
abundant  crops  of  potatoes. 

Chloride  of  sodium  (common  salt)  has  generally  a  fertiliz- 
ing influence  on  high  or  sheltered  lands  situated  at  a  distance 
from  the  sea. 

Kelp  (the  ash  of  sea  weeds)  and  wood  ash  are  well  known 
to  have  a  beneficial  action  on  all  kinds  of  soils. 

Chloride  of  potassium  (the  residue  of  the  nitre  refiners)  is 
sometimes  used  as  a  dressing  for  grass  land. 

Nitrates  of  potassa  and  soda.  —  These  have  been  found 
especially  beneficial  to  young  plants.     The  nitric  acid  which 
they  contain  supplies  nitrogen  to  the  vegetable,  and  the  po- 
tassa and  soda  are  equally  fertilizing ;  applied  at  the  rate  of 
44 


518  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

about  1  cwt.  per  acre,  they  promote  the  growth  of  young  corn 
and  grass. 

Gas  liquor  contains  a  large  quantity  of  ammonia  ;  it  there- 
fore forms,  when  diluted  with  five  or  six  times  its  weight  of 
water,  a  superior  manure  for  grass  lands  or  crops  generally. 
Sulphuric  acid,  or  gypsum,  is  sometimes  added,  to  fix  the  am- 
monia in  the  gas  liquor. 

101.  Special  manures.  —  As  plants  differ  in  their  composition, 
so  different  plants  evince  a  predilection  for  different  kinds  of 
food.      Ammonia,   nitrate   of   soda,   and   lime   promote    the 
growth  of  all  plants.     Lime,  especially  in  well-drained  soils, 
tends  to  bring  the  fruit  or  seeds  of  plants  to  perfection,  and 
thus  to  bring  in  an  early  harvest.     Gypsum  promotes   the 
growth  of  red  clover,  and  phosphate  of  magnesia  has  a  similar 
effect  upon  potatoes ;  and  so  on  to  other  cases. 

The  specific  action  of  particular  manures  on  the  growth  of 
certain  plants  is  a  remarkable  and  interesting  fact.  Even  cer- 
tain manures-  promote  the  development  of  particular  parts  of 
the  plant ;  thus,  for  example,  manganese  added  to  the  soil 
improves  the  flowers  of  the  rose  bush. 

102.  Mixed  saline  manures.  —  A  mixture  of  lime  and  com- 
mon salt  is  recommended  as  an  excellent  manure.     A  mixture 
of  sulphate  and  nitrate  of  soda,  as  a  top  dressing,  has  been 
found  to  produce  remarkable  effects  on  the  growth  of  potatoes  ; 
and  so  on  to  other  cases.     It  appears  that  the  application  of 
mixed  saline  substances  is  calculated  to  produce  more  benefi- 
cial results  than  when  these  substances  are  used  alone.     Hence 
it  is  that  guano  (which  contains  several  saline  substances)  is 
found  to  act  so  beneficially  on  almost  every  kind  of  crops. 

ROTATION    OF     CROPS. 

103.  The  composition  of  soils  should  have  a  relation  to  the 
kind  of  plants  which  they  are   intended  to  grow.     When  a 
particular  species  of  plant  has  been  grown  for  a  length  of 
time  on  a  soil,  that  soil  becomes  exhausted  of  the  inorganic 
matter  adapted  to  the  growth  of  that  particular  plant.     Now, 


EXPERIMENTAL  CHEMISTRY.  519 

different  plants  extract  from  soils  different  proportions  of  the 
inorganic  matter  contained  in  them.  Hence  a  succession  of 
crops  of  different  vegetables  may  be  raised  upon  the  same 
soil,  when  two  successive  crops  of  the  same  vegetable  could 
scarcely  be  reared.  Thus  barley  grows  well  after  a  crop  of 
turnips,  oats  after  a  crop  of  grass,  wheat  after  crops  of  beans 
and  potatoes.  The  following  is  a  specimen  of  a  six  years' 
rotation  of  crops  :  — 

1.  Wheat;  2.  Turnips;  3.  Barley;  4.  Seeds;  5.  Oats; 
6.  Potatoes. 

The  following  general  rule  should  be  observed  in  the  choice 
of  the  rotation  of  crops,  viz.,  plants  which  require  chiefly -the 
same  kind  of  food  should  not  be  grown  in  succession;  thus 
plants  which  are  grown  for  their  roots  grow  best  after  those 
which  are  grown  for  their  seeds. 

Clover  adds  fertility  to  the  soil ;  and  hence  an  abundant 
crop  of  corn  may  be  obtained,  after  a  crop  of  clover.  In  this 
way  the  use  of  clover  has,  to  a  great  extent,  superseded  the 
system  of  fallowing. 

104.  Fallow.  —  When  land  has  been  exhausted  by  a  succes- 
sion of  crops,  its  exhausted  resources  are  resuscitated  by  ma- 
nuring, and  allowing  it  to  lie  dormant,  exposing  it  at  the  same 
time  (by  ploughing,  &c.)  to  the  action  of  the  air  and  moisture. 

105.  Irrigation.  —  When  water  is  allowed  to   remain   on 
land,  it  is  injurious  to  vegetation  ;  but  the  occasional  flow  of 
water  over  the  surface  of  lands,  as  in  our  irrigated  meadows, 
carries  with  it  various  fertilizing  substances. 


QUESTIONS. 


INTRODUCTION. —  1.  Into  what  four  classes  are  the  laws  of  nature 
divided  ?  What  do  the  laws  of  physics  govern  ?  By  what  terms  are 
the  four  great  physical  truths  expressed  ?  What  phenomena  do  solids 
exhibit  ?•  What  do  liquids  ?  What  airs  ?  What  imponderables  ? 

MECHANICS.  —  5.  What  is  mechanics'?  Statics'?  Dynamics?  Hy- 
drostatics? Hydrodynamics?  Define  matter;  mass;  density. 

7.  What  is  motion  ?  When  is  motion  uniform  ?  When  accelerated  ? 
When  retarded  ?  What  is  velocity  ?  What  is  momentum  ?  What  is 
force  ?  Different  kinds  of  force  ?  What  is  meant  by  ponderable  and 
imponderable  bodies  ?  How  are  forces  known  to  us  ? 

13.  How  are  the  properties  of  matter  divided  ?  Which  are  primary 
properties  ?  Which  are  the  secondary  properties  ?  Define  extension ; 
impenetrability.  What  are  compressibility  and  expansibility  ?  What  is 
divisibility?  Cohesion?  Elasticity?  Mobility?  Inertia?  Gravity? 

24.  What  is  meant  by  the  attraction  of  gravitation  ?     What  is  the 
first  law  of  attraction  ?     The  second  law  ?     Summary  law  ?     On  what 
does  the  force  of  gravity  at  any  place  depend  ?   How  much  velocity  does 
a  falling  body  acquire  in  a  second  of  time  ?     State  the  law  of  increase. 

25.  What  is  the  centre  of  gravity  ?     What  is  meant  by  the  line  of 
direction  ?   How  does  the  line  of  direction  govern  the  stability  of  a  body  ? 
What  effect  has  the  elevation  of  the  centre  of  gravity  above  the  base  ? 

26.  What  is  the  first  law  of  motion  ?     What  are  the  obstacles  to  mo- 
tion ?    ^Name  the  second  law  of  motion.     What  is  the  parallelogram  of 
motion  ?     What  is  the  parallelogram  of  forces  ?     Give  the  third  law  of 
motion.     By  what  is  the  intensity  of  the  action  of  any  force  estimated  ? 

27.  Give  the  law  of  descent  of  falling  bodies.     How  is  motion  affected 
in  a  body  projected  vertically  upwards  ?      28.    What  is  a  parabola  ? 
29.  Give  the  law  of  vibration  of  a  pendulum. 

30.  How  many  and  what  forces  are  necessary  to  produce  motion  round 
a  centre  ?     What  is  centrifugal  force  ?     By  what  is  it  counteracted  ? 

31.  On  what  does  the  amount  of  work  done  by  an  agent  depend  ? 
What  is  the  unit  of  work,  as  adopted  in  this  country  ?     What  is  the  law 
of  labor  in  raising  a  body  in  opposition  to  gravity  ?     What  is  the  esti- 
mate of  a  horse  power 

32.  What  is  the  object  of  machinery  ?     Of  what  is  work  the  product  ? 
Name  some  of  the  active  agents  of  nature.     What  is  a  fundamental 
axiom  in  mechanics?     What  law  is  founded  on  the  principle   of  the 
equality  of  work  ?    To  what  is  the  advantage  gained  by  a  machine  equiv- 
alent ?     How  is  the  principle  of  virtual  velocities  commonly  expressed  ? 

(520) 


QUESTIONS.  521 

33.  Which  are  the  simple  mechanical  powers  ?  "What  is  the  lever  ? 
How  many  kinds  ?  Describe  each  kind.  35.  What  is  the  wheel  and 
axle  ?  36.  Describe  a  windlass.  37.  By  what  means  may  the  motion  of 
one  wheel  be  transmitted  to  others  ?  39.  What  is  a  capstan  ?  40.  De- 
scribe a  gib  crane. 

41.  What  is  a  pulley  ?     Of  what  kinds  ?     Mention  some  of  their  uses. 

44.  What  is  an  inclined  plane  1     How  is  its  advantage   estimated  ? 

45.  What  is  a  wedge  ?     Its  uses.    46.  Describe  a  screw.     How  is  the 
screw  regarded  "?    47.  Chief  uses  of  the  screw  ? 

48.  By  what  means  may  motion  be  communicated  from  one  axis  to 
another :?  What  is  a  train  of  wheels  ?  What  is  the  purpose  of  crown, 
bevelled,  and  face  wheels  ?  Describe  the  rack  and  pinion. 

STEAM  ENGINE.  —  Use  of  the  crank  and  connecting  rod  ?  Of  the  fly 
wheel  ?  3.  Describe  the  sun  and  planet  wheel.  4.  What  is  the  use  of 
Watt's  parallel  motion  ?  5.  What  is  an  eccentric  wheel  ?  6.  The  gov- 
ernor "?  7.  Describe  the  steam  boiler.  The  safety  valve.  9.  Use  of  the 
steam  gauge  1  10.  Use  of  the  water  gauge  ?  11.  The  water  regulator? 
What  are  the  respective  peculiarities  of  the  high  and  low  pressure  engine  ? 

HYDROSTATICS  AND  HYDRAULICS.  —  1.  What  is  hydrostatics  ?  What 
is  hydraulics  ~1  2.  How  do  fluids  differ  from  solids  ?  3.  Difference  be- 
tween liquids  and  gases  ?  5.  What  is  the  first  law  or  property  of  fluid 
bodies  ?  Illustrate  this.  The  second  law  ?  The  third  ? 

13.  Give  the  rule  for  finding  the  amount  of  pressure  upon  the  bottom 
of  a  vessel  containing  water  ?  14.  Upon  what  does  the  pressure  on  the 
bottom  of  a  vessel  depend  ? 

15.  Show  the  upward  pressure  of  water  by  an  experiment.  18.  Men- 
tion some  fact  in  nature  illustrating  this. 

19.  Rule  for  finding  the  pressure  on  the  side  of  a  vessel  ?     Illustrate. 

21.  What  is  the  centre  of  pressure  in  a  vessel  of  water  ? 

22.  What  is  the  specific  gravity  of  a  body  ?     What  is  used  as  the  stan- 
dard of  comparison "? 

23.  What  determines  the  sinking  or  floating  of  a  body  ? 

24.  What  is  the  first  of  the  laws  regulating  the  pressure  of  fluids  on 
solids  immersed  in  them  ?     The  second  law  ? 

27.  Describe  the  hydrostatic  balance.    Give  the  rule  for  determining 
the  specific  gravity  of  a.  solid  body. 

28.  Rule  for  finding  the  specific  gravity  of  a  liquid  ? 

33.  Chief  use  of  the  hydrometer  ? 

34.  Why  does  an  iron  vessel  float  in  water  ? 

35.  What  is  requisite  in  order  that  a  body  may  float  with  stability "? 
Illustrate  this. 

36.  Explain  what  is  meant  by  capillary  attraction. 

37.  What  is  the  law  of  attraction  in  capillary  tubes  ?    What  is  the 
effect  of  oiling  the  tube  ? 

39.  Explain  the  meaning  of  cndosmose  and  exosmose. 

Hydraulics.  —  41.  State  the  law  of  the  efflux  of  water  through  an  aper- 
ture in  a  vessel.  42.  To  what  is  the  velocity  proportioned  ?  By  what 
is  this  rule  modified  in  practice  ?  What  difference  in  effect  is  produced 
between  the  use  of  a  short  pipe  and  a  simple  aperture  ?  Explain  this. 

PNEUMATICS. —  1.  What  is  pneumatics?  2.  Height  of  the  atmos- 
phere ?  Why  is  air  believed  to  be  material  ? 

3.  Its  resistance  to  motion  ? 

4.  Its  impenetrability  ? 

5.  Evidence  of  its  weight  ? 

44* 


522          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

7.  To  what  is  the  atmospheric  pressure  equivalent  ?  8.  How  much 
pressure  on  a  square  inch  ?  How  high  a  column  of  mercury  will  the 
pressure  of  the  atmosphere  sustain  ?  Of  water  ?  9.  Show  the  utility  of 
the  atmospheric  pressure  on  our  bodies.  10.  Explain  the  construction 
and  use  of  the  barometer.  11.  Why  does  water  continue  to  flow  through 
a  siphon  when  put  in  operation  ? 

12.  Explain  the  cause  of  intermitting  springs. 

14.  By  what  is  the  elasticity  of  air  increased  ?     State  the  law  of  elas- 
ticity of  the  air. 

15.  What  relation  is  there  between  the  density  of  the  air  and  its  height 
above  the  level  of  the  sea  ? 

16.  How  are  bodies  affected  by  heat?     What  class  of  bodies  are  most 
susceptible  of  this  action  ?     What  is  the  cause  of  wind  ? 

19.  What  is  the  purpose  of  an  air  pump?  Can  a  perfect  vacuum  be 
produced  by  an  air  pump  ?  Why  does  water  boil  at  a  lower  temperature 
on  a  mountain  than  at  the  sea  level  1 

21.  Describe  the  construction  of  a  common  lifting  pump.   To  what  limit 
may  a  column  of  water  be  raised  by  the  ascending  piston  ? 

22.  Explain  the  action  of  a  common  forcing  pump. 

23.  Of  what  use  is  an  air  chamber  in  a  forcing  pump  ? 

24.  Advantage  of  a  double-acting  pump  ?    25.  How  is  the  fire  engine 
constructed  ? 

30.  What  is  meant  by  the  diffusion  of  gases'?  31.  Its  use  in  nature? 
32.  What  do  you  know  of  the  liquefaction  of  gases  ? 

Acoustics.  —  33.  How  is  sound  conveyed  to  the  ear  ?     When  is  sound 

heard  ?     Effect  of  quickly-repeated  impulses  ?    What  constitutes  a  tone  ? 

What  effects  are  produced  when  a  sonorous  body  is  struck  ?     34.  What 

'property  is  essential  to  a  sonorous  body?     On  what  does  the  pitch  of  its 

tone  depend  ?     What  constitutes  a  noise  ? 

35.  What  is  necessary  to  the  transmission  of  sound  ?  36.  On  what 
does  the  greater  conducting  power  of  the  air  depend  ?  37.  When  a  gun 
is  fired  what  difference  of  velocity  is  noticed  between  the  flash  and  the 
sound?  At  what  rate  does  sound  travel  ?  38.  Is  there  any  better  medium 
than  air  for  transmitting  sound? 

39.  How  is  sound  reflected  ? 

40.  What  is  the  cause  of  echoes  ? 

41.  How  may  sound  be  magnified  ?  *V    .      ,, 

44.  How  are  winds  produced  ?      Explain  the  action  ot  land  and  sea 

45.  What  are  the  three  general  classes  of  winds  ?     Explain  the  trade 
winds;  the  monsoons,  the  variables ;  the  sirocco  and  simoom.    46.  What 
velocity  constitutes  a  gentle  breeze  ?    A  brisk  gale  ?     A  high  wind  ?    A 
hurricane  ?    Law  of  increase  ? 

47.  Why  does  a  balloon  ascend  ? 

LIGHT.  —  1.  Relation  of  light  to  the  eye?  2.  Sources  of  light? 
3.  What  are  non-luminous  bodies  ?  4.  Division  of  bodies  with  relation 
to  light  ?  What  is  the  characteristic  of  each  ? 

5.  Velocity  of  light  ? 
Direction  of  light  ? 

6.  Law  of  intensity  of  light  ? 

7.  Two  remarkable  laws  of  light  ?    Exemplify  each. 

8.  Describe  the  two  theories  of  the  nature  of  light. 

9.  Law  of  reflection  ? 

10.  Effect  on  light  in  passing  from  one  medium  to  another?     Wnat 
is  meant  by  the  interference  of  light  ?     The  diffraction  ? 


QUESTIONS.  523 

11.  State  the  three  kinds  of  imriprs.    By  what  law  are  mirrors  gov- 
erned ? 
'     12.  What  is  the  general  effect  of  concave  mirrors  ? 

13.  Where  is  the  principal  focus  of  a  convex  mirror  ?    Why  called  the 
virtual  focus  ?     What  is  the  general  effect  of  convex  mirrors  ? 

14.  State  the  law  in  relation  to  the  refraction  of  light.    On  what  does 
the  higher  refractive  power  of  a  medium  depend  ?     What  follows  when 
a  ray  of  light  passes  from  one  medium  to  another  of  different  density  ? 
15.  What  when  passing  through  a  plate  of  glass  1   16.  What  when  pass- 
ing into  and  out  of  a  prism  ?     18.  Describe  the  several  forms  of  lenses ; 
the  parts  of  a  lens. 

19.  What  is  the  focal  distance  of  a  double  convex  lens,  according  as 
the  incident  rays  are  parallel,  divergent,  or  convergent  ?  20.  Of  a  plano- 
convex lens  ?  22.  Of  a  double  concave  lens  ? 

23.  What  effect  has  a  convex  lens  upon  the  apparent  size  of  an  object 
seen  through  it  ? 

24.  What  are  diminishing  glasses  ? 

25.  Effects  of  spherical  aberration  ?     The  remedy  ? 

27.  Remarks  on  the  eye  ?     Describe  its  various  parts.     What  is  the 
cause  of  short-sightedness  ?     What  of  long-styhtedness  ?     How  do  we  judge 
of  the  actual  size  of  an  object  seen  at  a  distance  ?     How  of  the  distance  ? 

What  is  meant  by  the  visual  or  optical  angle  ?  On  what  does  the  size 
of  the  image  on  the  retina  depend  ? 

28.  What  is  the  purpose  of  a  microscope  ?     How  is  this  effect  pro- 
duced ? 

31.  What  is  the  purpose  of  a  telescope  ?     What  two  kinds  are  used  ? 

40.  What  is  a  camera  obscura  ? 

41.  What  is  a  magic  lantern  ? 

42.  Give  a  description  of  the  stereoscope. 

Phenomena  of  Color.  —  43.  Of  what  is  a  ray  of  solar  light  composed  ? 
How  may  this' be  proved1?  44.  By  whom  was  solar  light  first  decom- 
posed ?  45.  What  distinct  properties  exist  in  the  solar  spectrum  ? 
Where  is  the  most  luminous,  portion  located  ?  Wherev  the  most  heating 
portion  ?  Where  the  greatest  chemical  intensity  1  46.  Which  are  the 
three  fundamental  colors  ?  What  do  you  understand  by  a  complementary 
rat/? 

47.  On  what  principle  does  the  rainbow  depend  ? 

48.  How  is  the  phenomenon  termed  the  mirage  accounted  for  ? 

49.  What  are  halos  ? 

50.  What  is  doubte  refraction,  and  how  produced 

51.  Explain  the  polarization  of  light  by  reflection. 
53.  Explain  the  polarization  of  light  by  refraction. 

HEAT.  —  55.  Remarks  on  heat  ?  Free  or  sensible  heat  ?  Tempera- 
ture ?  Latent  heat  ?  What  does  the  term  caloric  express  ?  Heat  and 
cold  ?  To  what  laws  is  calorie  subject  1  What  is  one  of  the  most  strik- 
ing effects  of  heat  ? 

56.  Illustrate  the  expansion  of  liquids  by  heat.     Air.     Solids. 

57.  What  are  some  of  the  sources  of  heat  ?     With  what  is  a  change 
of  volume  in  a  body  always  attended  ? 

58.  Give  illustrations  of  good  and  bad  conductors  of  heat. 

59.  By  what  principle  is  the  radiating  power  of  surfaces  regulated  ? 

60.  Effect  of  heat  on  liquids,  and  of  cold  on  vapors  ? 
What  is  evaporation  ? 

What  is  meant  by  the  dew  point  ?    When  is  the  air  said  to  be  satu- 


524          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

rated  with  moisture  ?  When  is  evaporation  carried  on  most  rapidly  ? 
What  causes  fog,  mist,  dew,  &c.  ?  TJpon  what  does  the  absolute  quan- 
tity of  moisture  which  the  air  will  sustain  depend  ? 

61.  Is  heat  or  cold  produced  when  certain  substances  melt?     Give 
examples. 

62.  What  allowance  should  be  made  in  the  construction  of  large  me- 
tallic structures  ? 

63.  Explain  the  principle  of  the  compensation  pendulum. 

64.  What  is  a  thermometer'?     Different  kinds?    Freezing  tempera- 
ture ?    Boiling  temperature  ? 

65.  In  what  various  ways  is  heat  propagated  ? 

66.  In  what  manner  is  radiant  caloric  thrown  off  from  a  surface  ?     On 
what  does  the  reflecting  power  of  substances  depend  ?     Which  are  the 
best  reflectors  ?     State  the  general  rule  of  absorption.     What  is  meant 
by  diathermanous  and  athermanous  bodies  ? 

Upon  what  does  the  power  of  a  body  to  transmit  heat  depend  ? 
What  is  Leslie's  law  of  radiant  heat  ? 

67.  According  to  what  does  the  propagation  of  heat  in  liquids  vary  ? 
What  follows  when  heat  is  applied  to  the  surface  of  a  liquid  ? 

At  what  temperature  does  water  attain  its  greatest  density  ? 

68.  Remark  on  the  heat  of  the  ocean  ?     Why  does  ice  always  form  at 
the  surface  of  the  water  ? 

69.  Remark  on  the  heat  of  the  atmosphere. 

71.  On  what  does  the  rate  of  conduction  in  bodies  depend? 

72.  To  what  is  the  amount  of  free  caloric  in  two  different  quantities 
of  the  same  substance  proportional  ?     What  of  equal  weights  of  dissimi- 
lar substances  ? 

What  is  meant  by  the  specific  heat  of  a  body  ?  What  relation  has  the 
density  of  a  body  to  its  capacity  for  heat  ?  What  is  a  calorimeter  ?  What 
other  change  attends  a  change  of  volume  in  a  body  ? 

73.  What  is  fusion  ?     What  is  vaporization  ?    Evaporation  ? 

74.  What  number  of  degrees  represents  the  latent  heat  of  steam  ? 

76.  What  is  the  crt/ophorus  ? 

77.  What  is  an  hygrometer  ? 

78.  How  does  the  air  become  warm  ?     Which  strata  of  the  atmos- 
phere are  warmest,  and  why  ?     What  change  takes  place  after  sunset  ? 
When  does  the  earth  radiate  heat  most  freely  ?     When  is  the  deposition 
of  dew  most  copious  ?     Why  does  the  gardener  cover  tender  plants  with 
straw  at  night  ?  What  are  considered  as  the  great  causes  of  rain  ?    Uses 
of  evaporation  and  condensation  ?      How  may  the  suspension  of  the 
particles  of  moisture  in  the  clouds  be  accounted  for  ? 

ELECTRICITY.  —  1.  What  is  electricity?  What  are  some  of  the 
means  for  generating  it  ? 

2.  What  are  the  fundamental  facts  of  electricity  ?     What  are  electrics  ? 
What  fact  with  regard  to  an  electrified  body  was  first  made  known  by 
Newton  ? 

3.  General  remarks  on  conductors  and  non-conductors  ?    When  is  a 
body  said  to  be  insulated  ?     What  substances  are  most  commonly  used 
as  insulators  ?     What  effect  has  dampness  upon  insulators  ?     Which  is 
the  best  insulator,  and  why  ?     Which  arc  the  best  conductors  of  electri- 
city ?     Is  the  atmosphere  a  conductor  or  a  non-conductor  ?     In  what 
condition  is  rarefied  air  '. 

4.  What  is  an  electroscope  ? 

5.  How  many  kinds  of  electricity  are  there  ?     What  is  the  law  of 
attraction  and  repulsion  ? 


QUESTIONS.  525 

6.  Docs  electricity  pass  from  one  part  of  the  surface  of  a  non-conductor 
to  another,  or  does  it  remain  stationary  ? 

What  is  the  relative  condition  of  the  electricity  of  the  rubber  and  that 
of  the  body  rubbed  ? 

8.  Explain  the  use  of  the  terms  positive  and  negative. 

What  is  the  theory  of  tioo  fluids  ?  The  terms  used  ?  Of  what  use  is 
theory  in  this  subject  ? 

9.  What  is  meant  by  conduction  ?     By  induction  ? 

12.  Name  the  principal  parts  of  an  electrical  machine,  and  their  uses. 
15.  What  are  the  usual  appendages  to  the  electrical  machine  ? 

18.  Name  some  facts  relating  to  the  electric  spark. 

19.  On  what  does  the  intensity  of  the  electric  light  depend  ? 

20.  What  is  the  electric  recoil? 

23.  Upon  what  circumstances  does  the  intensity  of  the  electricity  trans- 
mitted by  the  electrophorus  depend  ? 

26.  What  is  disguised  electricity  ? 

27.  U8e  of  the  condenser  ? 

28.  Explain  the  difference  between  an  electroscope  and  an  electrom- 
eter. . 

30.  Give  a  description  of  an  electrical  battery.   How  is  it  discharged  % 

31.  How  is  the  intensity  of  the  electricity  determined  by  a  discharging 
electrometer  ? 

34.  Mention  some  of  the  most  common  physiological  effects  of  elec- 
tricity. What  is  meant  by  an  electrical  shock  ? 

36.  Mention  some  of  the  chemical  effects  of  electricity. 

37.  How  is  the  electric  fluid   distributed  with  regard  to  conductors  ? 
What  instrument  shows  this  ?      Upon  what  does  the  intensity  of  the 
electricity  depend  in  a  conductor  ? 

38.  By  whom  was  the  identity  of  electricity  and  lightning  discovered  ? 
Mention  their  points  of  agreement.     Describe  Franklin's  experiment. 

39.  When  is  atmospheric  electricity  generally  positive  ?     When  neg- 
ative?    Where  most  intense1?     Its  intensity  in  winter  compared  with 
that  in  summer  ? 

What  is  the  character  of  the  electricity  in  rain  drops  in  a  north  and  in 
a  south  wind  ?  Characterize  the  electricity  of  the  earth  and  the  higher 
regions  of  the  atmosphere.  How  often  does  aerial  electricity  attain  a 
maximum  and  minimum  condition  ?  Describe  its  progress  from  one 
extreme  to  the  other. 

40.  Which  are  the  most  common  electrometeors  ? 

41.  Give  an  account  of  the  aurora  borealis.    42.  Of  the  waterspout. 
43.  By  what  various  modes  is  electricity  generated  ? 
MAGNETISM. —  1.   What  metal  is  alone  atti'acted  by  magnetism? 

What  are  the  substances  possessing  the  magnetic  property  called  ? 

Of  what  two  kinds  are  magnets  ?  What  are  natural  magnets  ?  Ori- 
gin of  the  name  ?  What  are  artificial  magnets  ?  Mention  the  different 
sorts.  What  constitutes  a  magnetic  battery  ? 

Where  in  a  bar  magnet  docs  the  power  chiefly  reside  ?  How  are  the 
extremities  of  a  magnet  distinguished  ? 

What  is  one  of  the  most  remarkable  properties  of  the  magnet  ? 

2.  Show  the  reciprocal  attraction  between  a  magnet  and  iron.  What 
effect  is  produced  by  interposing  wood,  glass,  or  copper  between  the 
magnet  and  iron  ?  How  is  magnetism  distributed  throughout  a  mag- 
netized bar  ?  Where  are  the  poles  and  the  neutral  point  respectively 
situated  ? 


526          NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

3.  What  is  the  directive  polarity  of  the  magnetic  needle  ? 

What  is  the  magnetic  meridian  ?  Does  it  coincide  with  the  geograph- 
ical meridian  ?  What  is  the  difference  between  them  called  ?  Is  the 
declination  uniform  in  all  places  ? 

5.  State  the  law  of  magnetic  attraction.     How  are  the  poles  distin- 
guished ? 

6.  What  is  the  effect  of  breaking  a  magnet  ? 

7.  State  the  theory  of  magnetism.     Mention  some  of  the  phenomena 
of  magnetism. 

8.  What  is  the  law  of  the  attractive  force  of  the  magnet  ? 

9.  What  is  magnetic  conduction  ?     What  is  magnetic  induction  ? 

12.  Describe  the  property  called  the  dip  of  the  needle.  Is  the  angle  of 
the  needle's  dip  uniform  at  all  places  on  the  earth  ? 

1 6.  What  are  the  requisites   in  the  quality  of  steel  for  making  mag- 
nets '?       What  is  necessary  in  the  form  and   dimensions  of   artificial 
magnets  ? 

17.  How  may  a  soft  iron  bar  be  rendered  magnetic  ? 

Which  is  the  most  powerful  means  of  rendering  bodies  magnetic  ? 

18.  How  is  the  earth  to  be  regarded  in  order  to  account  for  the  direc- 
tive  and  dipping  properties  of  the  needle  ?  Which  of  the  magnetic  poles 
of  the  earth  is  the  positive  pole  ?  What  is  the  situation  of  the  terrestrial 
magnetic  poles  ? 

What  are  isoclinic  lines  ?     With  what  do  they  coincide  ? 

1.9.  To  what  variations  are  the  earth's  magnetic  powers  subject ? 
How  indicated  ?  Characterize  the  regular  variations.  Which  of  the  sec- 
ular variations  has  been  most  observed  ?  With  what  are  the  irregular 
magnetic  variations  connected  ? 

22.  What  is  Ampere's  theory  of  magnetism  ? 

VOLTAIC  ELECTRICITY. — "l.  How  is  voltaic  electricity  produced? 
By  whom  was  it  first  observed,  and  under  what  circumstances  ?  How 
did  Galvani  account  for  it  ?  How  did  Volta  account  for  it  ? 

2.  Describe  the  voltaic  pile.  Describe  the  Couronne  de  Tasses,  or  Vol- 
ta's  battery. 

8.  What  is  a  voltameter  ?     What  three  kinds  are  in  use? 

9.  What  is  considered  as  one  of  the  most  important  features  of  voltaic 
electricity  ?     What  remarkable  fact  is  connected  with  voltaic  decomposi- 
tion ?     How  are  the  constituents  of  water  disposed  after  decomposition  ? 
What  system  is  based  upon  this  fact  ?     What  is  the  electrical  state  of 
the  elements  which  are  attracted  by  the  positive  pole  ? 

10.  Upon  what  do  the  arts  of  electrotyping  and  electroplating  depend  ? 

12.  How  may  decomposition  be  impeded  or  arrested  by  voltaic  elec- 
tricity ? 

13.  Give  instances  of  the  luminous  and  heating  effects  of  voltaic  elec- 
tricity.   Upon  what  does  the  temperature  to  which  a  conducting  wire 
will  be  raised  by  a  battery  depend  ? 

Upon  what  does  the  calorific  effect  depend  ? 

14.  State  some  facts  in  relation  to  the  physiological  effects  of  voltaic 
electricity.     Upon  what  do  these  effects  seem  to  depend  ? 

ELECTRO-DYNAMICS.  —  1.  Explain  the  construction  of  the  right  and 
left-handed  helices.  How  should  helix  wires  be  prepared  ?  State  the 
facts  in  connection  with  a  needle  magnetized  by  a  right-handed  and  a 
left-handed  helix. 

2.  W«bat  is  an  electro-magnet  ?  How  long  will  an  electro-magnet  retain 
the  magnetic  property  ? 


QUESTIONS. 


527 


7.  State  the  five  general  laws  of  electro-dynamic  action. 

10.  State  the  two  laws  according  to  which  electric  currents  act  upon 
each  other. 

What  are  the  laws  respecting  angular  currents  ? 

Electro-Dynamic  Induction.  — 12.  Who  first  discovered  the  laws  of 
electro-dynamic  induction  ?  What  did  he  demonstrate  ?  When  does 
the  induction  of  the  current  act  ? 

16.  What  is  thermo-electricity  ?     Give  examples. 

18.  What  are  dia-magnetic  bodies  ? 

19.  Describe  the  electro-magnetic  telegraph. 

20.  What  are  the  peculiarities  of  Morse's  telegraph?    21.  Of  Bain's  ? 
22.  Of  House's  ? 

CHEMISTRY.  —  1.  Of  what  docs  the  science  of  chemistry  treat  ?  How 
many  elementary  substances  are  there  ?  Difference  between  a  simple 
and  a  compound  substance  ?  2.  How  are  the  elementary  substances 
divided  ?  Name  those  of  each  class. 

4.  Mention  the  various  kinds  of  attraction. 

9.  How  does  chemical  affinity  differ  from  all  other  kinds  of  attraction  ? 
By  what  are  all  chemical  changes  produced  ?     When  does  combination 
take  place  ?     When  decomposition  ? 
'  10.  Is  any  thing  in  nature  ever  destroyed  or  annihilated  ? 

1 1 .  Characterize  acids.    Alkalies. 

12.  What  is  a  solution  ?     How  is  the  process  of  solution  accelerated  ? 

13.  How  is  carbon  obtained  ?     What  is  the  product  of  the  combustion 
of  charcoal  ?     Characterize  carbonic  acid  gas. 

14.  Describe  hydrogen.    Where  found  ?     Characterize  hydrogen. 

15.  Composition  of  the  atmosphere  ?    What  takes  place  in  the  process 
of  breathing?      Characterize  oxygen.     Characterize  nitrogen.     Relation 
of  oxygen  to  plants  and  animals  ? 

16.  What  is  ammonia1?     Where  found? 

17.  What  is  nitric  acid? 

18.  Give  a  description  of  the  atmosphere.    What  are  its  constituents  ? 

19.  Where  is  native  sulphur  obtained  ?    With  what  metals  is  it  found 
in  combination  ?     Which  is  the  most  important  compound  of  sulphur  ? 

20.  Characterize  phosphorus. 

21.  Characterize  iodine.    From  what  is  it  obtained  ?     Chief  use  ? 

22.  Characterize  chlorine.     Use  of  chlorate  of  potassa  ? 

23.  Characterize  hydrochloric  acid. 

,  24.  From  what  are  potassa  and  soda  formed?   Why  called  fixed  alka- 
lies ?     In  what  does  potassa  largely  exist  ? 

25.  In  what  various  forms  does  lime  exist  ?    Its  relation  to  soils  ?   Its 
uses  in  agriculture  ?     What  is  the  metallic  base  of  lime  ? 

26.  Characterize  magnesia. 

27.  Characterize  alumina. 

Of  what  is  pure  clay  composed  ? 

28.  Characterize  silica.     What  is  its  base  ? 

29.  In  what  forms  does  iron  exist  in  nature  ? 

30.  In  what  states  does  copper  exist  in  nature  ? 

31.  Common  native  form  of  lead? 

32.  Most  common  salt  of  chrome? 

33.  In  what  state  is  mercury  found  ? 

34.  Remark  on  zinc. 

35.  Characterize  silver.    36.  Gold.    37.  Platinum. 

38.  What  is  the  doctrine  of  chemical  equivalents  ?     Give  examples. 


528  NATURAL    AND    EXPERIMENTAL    PHILOSOPHY. 

40.  What  is  taken  as  the  symbol  of  a  simple  substance  ?  What  does 
the  name  of  a  compound  indicate  ?  Examples  ? 

65.  Of  what  does  the  organic  part  of  plants  consist  ?     Of  what  the 
inorganic  ?    Is  their  proportion  uniform  ? 

66.  Effects  of  different  kinds  of  plants  upon  the  inorganic  matter  of 
soils  ?     State  facts  in  illustration  of  this. 

67.  What  shows  the  necessity  for  manuring  soils  ? 

68.  Which  are  the  most  abundant  compound  organic  substances  in 
plants  ?     Their  constituents  ?     By  what  circumstances  are  most  vege- 
table compounds  characterized  ?     What  are  these  distinct  compounds 
called  ? 

69.  What  is  a  singular  fact  with  regard  to  lignine,  starch,  gum,  and 
sugar  1 

What  is  catalysis  ? 

70.  What  does  fermentation  signify  ?     What  is  the  result  of  the  fer- 
mentation of  saccharine  matter  ? 

71.  What  is  acetous  fermentation  ?     What  purpose  does  yeast  serve  in 
fermentation  ? 

72.  Which  are  the  most  common  vegetable  acids  ? 

73.  Give  an  account  of  the  process  of  germination. 

74.  Which  parts  of  a  plant  are  essential  to  its  growth  ?    Describe  the 
trunk,  or  stem.    The  root.    The  leaf.    In  what  other  way  does  a  plant 
receive  carbonic  acid  ? 

75.  What  must  the  food  of  plants  contain  ?     What  substances  afford 
this  food  ?     Effects  of  light  and  heat  upon  plants  ? 

76.  Composition  of  soils  ?  Whence  is  the  organic  part  derived  ?  What 
is  humus  ? 

77.  Of  what  does  the  inorganic  part  of  soils  consist  ?     Characterize 
the  saline  soluble  substances ;  also,  the  earthy  insoluble  substances. 

78.  What  constitutes  a  sandy  soil  ?     A  toawy  soil  ?     80.  What  war/  ? 
Calcareous  soil  ? 

83.  Origin  of  soils  ? 

84.  State  some  of  the  mechanical  properties  of  soils.     85.  Effects  of 
heat  on  clay  and  peat  ?     86.  Of  what  advantage  is  the  absorbent  power 
of  clay  ?     87.  Necessity  for  warmth. 

89.  State  facts  with  reference  to  the  chemical  properties  of  soils. 

90.  Importance  of  a  due  proportion  of  the  essential  elements  of  a  fer- 
tile soil  ? 

91.  How  may  land  be  improved  ? 

92.  Remark  on  draining?     93.    On  ploughing?     94.   On  subsoil  and 
deep  ploughing  ? 

95.  Name  the  three  classes  of  manures.  96.  What  are  the  purposes 
of  vegetable  manures  ?  Remarks  on  green  manures  ?  On  dry  manures  ? 

98.  Characterize  animal  manures.  What  is  the  important  constituent 
of  guano  ?  Which  portion  of  manures  is  most  valuable  ?  Why  ? 

100.  Most  important  of  mineral  manures  ?    Mention  some  of  its  uses. 
What  other  minerals  are  used  as  manures  ? 

101.  Give  instances  of  the  use  of  special  manures.     102.  Advantages 
of  mixed  saline  manures  ? 

103.  Relation  between  the  soil  and  the  plant  to  grow  from  it  ?     Effect 
of  growing  a  particular  species  of  plant  on  a  soil  ?     Effect  of  a  succes- 
sion of  crops  ?     Example  ?     Rule  '* 

104.  What  is  meant  by  fallowing  * 

105.  Remark  on  irrigation  ? 

^ 

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